Posted: July 25th, 2024

Homework

California’s Changing
Climate 201

8

CALIFORNIA’S FOURTH

CLIMATE CHANGE
ASSESSMEN

T

Coordinating Agencies:

A Summary of Key Findings from California’s
Fourth Climate Change Assessment

Introduction to California’s Fourth Climate Change Assessment

C alifornia is a global leader in using, investing in, and advancing research to set proactive climate change
policy, and its Climate Change Assessments provide the scientifc foundation for understanding climate-
related vulnerability at the local scale and informing resilience actions. The Climate Change Assessments
directly inform State policies, plans, programs, and guidance to promote effective and integrated action to

safeguard California from climate change.

This capstone report presents key fndings from California’s Fourth Climate Change
Assessment (also referred to as the Fourth Assessment). It provides an overview of the
state of climate science while pointing out how the Fourth Assessment contributes to
better understanding the impacts of climate change and how to take action to become
more resilient.

To fnd out more about the
other components of the
Fourth Assessment, please visit:

www.ClimateAssessment.ca.gov

KEY
FINDINGS

ASSESSMENT FOUNDATION:
UPDATED CLIMATE PROJECTIONS AND DATA

SUMMARIES FOR REGIONS
AND

COMMUNITIES

STATEWIDE
SUMMARY

ORIGINAL RESEARCH TO
INFORM POLICY AND ACTION

CALIFORNIA’S CLIMATE CHANGE POLICY AND
THE FOURTH ASSESSMENT

While California is leading eforts to reduce greenhouse
gas emissions, the State must also proactively address
current and future impacts of climate change. Te Fourth
Assessment is part of
California’s comprehensive
strategy to take action based
on cutting-edge climate
research. Te Fourth
Assessment addresses critical
information gaps that
decision-makers at
the state, regional, and local levels need addressed in
order to protect California’s people, infrastructure,
natural systems, working lands, and waters.

PEOPLE

INFRA-
STRUCTURE

NATURAL
SYSTEMS

Built infrastructure systems

including changes in climate

so that they both survive climate-

can withstand changing
conditions and shocks,

conditions, while continuing
to provide critical services

People and communities can respond to changing
average conditions, shocks, and stresses in a manner
that minimizes risks to public health, safety, and the

economy; and maximizes equity and
protection of the most vulnerable

related events and thrive despite
and after these events.

Natural systems adjust and
maintain desirable ecosystem

characteristics in the face of
change.

The Fourth Assessment provides critical information that will enable more ambitious efforts
to support a climate-resilient California.

http:www.ClimateAssessment.ca.gov

Why Study Climate Change in California?

C alifornia is one of the most “climate-challenged” regions of North America; its historical climate is
extremely variable, and climate change is making extreme conditions more frequent and severe. California’s
temperatures are already warming, heat waves are more frequent, and precipitation continues to be highly
variable. Since its Third Climate Change Assessment in 2012, California has experienced several of the most

extreme natural events in its recorded history: a severe
drought from 2012-2016, an almost non-existent Sierra
Nevada winter snowpack in 2014-2015, increasingly
large and severe wildfres, and back-to-back years of the
warmest average temperatures.

California and the world need to rapidly reduce climate
pollution to avoid the worst efects of climate change.
We must also prepare for the continued acceleration of
climate impacts in the future. Te Fourth Assessment has
prepared information needed to reach these goals.

Te Fourth Assessment includes 3

3

State-funded
research projects and contributions from

11

externally-
funded researchers. Te State-funded projects include
the development of cutting-edge climate projections for
California. Te projections use a broader range of climate
models, emission scenarios, and simulations than previous
assessments, and included:

• Te development and use of a new technique that
provides spatial climate data that can be used at the
local to regional level.

• Improved understanding of additional climate vari-
ables, including relative humidity and wind speed,
and extremes like drought, heat waves, and heavy
precipitation events.

• More extensive simulations of wildfre to help visual-
ize increases in area burned.

• A more detailed set of sea-level rise projections that
incorporate recent research on ice sheet collapse in
West Antarctica.

These projections are critical tools
necessary to understand and plan for
climate impacts. They also inform research
into critical actions for resilience.

CATALYZING ACTION THROUGH NEW ONLINE RESOURCES

www.Cal-Adapt.org

The Fourth Assessment supported the development and expansion
of new and existing resources to directly support climate action.
Examples include:

Cal-Adapt is the State’s portal for the climate projections produced for
the Fourth Assessment, enabling data downloading and visualizations
of climate scenarios at the local level and wildfre projections for the
entire state.

www.Cal-Heat.org

Cal-Heat is a new tool funded by the Fourth Assessment to inform
local public health offcials’ initiatives to protect the public during
climate-exacerbated extreme heat events.

COASTAL STORM MODELING SYSTEM (COSMOS)

The CoSMoS model, partly funded by the Fourth Assessment, provides
information about the complex interplay of coastal dynamics and
climate change for California’s coast.

View updated CoSMoS results on these websites:

• Hazard Exposure Reporting and Analytics (HERA)

• Our Coast Our Future Flood Map

https://www.usgs.gov/apps/hera/

www.OurCoastOurFuture.org

The full suite of Fourth Assessment projects and other tools can be
found at: www.ClimateAssessment.ca.gov

3

http:www.ClimateAssessment.ca.gov

http:www.OurCoastOurFuture.org

https://www.usgs.gov/apps/hera

http:www.Cal-Heat.org

http:www.Cal-Adapt.org

How is California’s climate projected to change?

T
he Fourth Assessment produced updated climate projections that provide state-of-the-art understanding
of different possible climate futures for California. The science is highly certain that California (and the
world) will continue to warm and experience greater impacts from climate change in the future. While
the Intergovernmental Panel on Climate Change and the National Climate Assessment have released

descriptions of scientifc consensus on climate change for the world and the United States, respectively, the Fourth
Assessment summarizes the current understanding of climate impacts and adaptation options in California. The
greater detail provided by the Fourth Assessment supports efforts by individuals, businesses and communities to
prepare for and reduce the impacts of climate change.

CLIMATE IMPACT DIRECTION
SCIENTIFIC CONFIDENCE

FOR FUTURE CHANGE

TEMPERATURE WARMING Very High

SEA LEVELS RISING Very High

SNOWPACK DECLINING Very High

HEAVY PRECIPITATION EVENTS INCREASING Medium-High

DROUGHT INCREASING Medium-High

AREA BURNED BY WILDFIRE INCREASING Medium High

While most of these trends have been generally understood and expected since before California’s First Climate
Change Assessment in 2006, the Fourth Assessment provides new quantitative tools to understand and address these
impacts. Te updated results from the suite of Fourth Assessment models and analyses demonstrate the importance
of achieving global reductions in greenhouse gas emissions. 1

1 The phrase “if greenhouse gas emissions continue at current rates” refers to the Representative Concentration Pathway (RCP) 8.5. The phrase “if greenhouse

gas emissions are reduced at a moderate rate” refers to RCP4.5. The RCP4.

5

emissions level represents reduced emissions, but those reductions are not

suffcient to achieve the targets called for in the Paris Agreement. However, the RCP4.5 emissions scenario was used in many of the Fourth Assessment’s

studies.

4

If greenhouse

are reduced at a
moderate rate…

then California will

experience average daily

2.5°F
from 200

6

to
2039.

4.4°F
from 2040 to
2069.

5.6°F
from 2070
to 2100.

gas emissions…
continue at
current rates…

high temperatures that

are warmer than the

historical average by…

2.7°F
from 2006
to 2039.

5.8°F
from 2040
to 2069.

8.8°F
from 2070
to 2100.

While the averages of daily maximum temperatures
over an entire year are easily understood, in many ways
this indicator obscures the risks from extreme weather
events due to changing climate. For example, the number
of extreme heat days will increase exponentially in
many areas.

Projections developed for the Fourth Assessment do
not show a consensus in the overall trend in yearly
precipitation, but they do have increasing variability
in precipitation. However, across all the simulations,
higher temperatures lead to dryer conditions because of
increasing evaporation and plant stress. With increased
numbers of dry days, several of the models indicated
an increased occurrence of dry years and strings of
dry years resulting in more frequent and more intense
droughts. At the same time that most of the simulations
had more dry days, there was also a tendency for
increased precipitation on very wet days, so that the risk
of foods caused by large storms will increase, sometimes
occurring in bursts over several weeks.

he Paris Agreement brought, for the frst time, all
nations of the world together around the common
cause of limiting global average temperature

warming to 2°C [3.6˚F] or less (1.5°C [2.7˚F]) above
pre-industrial levels. A Fourth Assessment study reports
estimated climate impacts to California assuming global
compliance with the Paris goals, fnding that impacts
in California would be substantially reduced. However,
California still needs to prepare, at a minimum, for
signifcant unavoidable impacts that would occur even
if global average temperate rise is limited to 1.5°C, and
adopt precautionary adaptation policy to protect against
impacts from higher emissions scenarios.

T

y 2050, the average water supply from
snowpack is projected to decline to 2/3 from
historical levels. If emissions reductions do not

occur, water from snowpack could fall to less than
1/3 of historical levels by 2100.

B
EXTREME HEAT DAYS PER YEAR IN DOWNTOWN FRESNO

(Days exceeding 106.6°F)

1961 – 2005

4

2050 – 209

9

26
if greenhouse
gas emissions

are reduced at a
moderate rate

2050 – 2099

43
if greenhouse
gas emissions
continue at
current rates

5

Sea-level rise is virtually certain to increase beyond the Increasing acreage burned by wildfre is associated with
6 inches that much of California experienced in the past increasing air temperatures. One Fourth Assessment
century, but there are important questions involving model suggests large wildfres (greater than 25,000 acres)
how fast and how extreme the rates of sea-level rise will could become 50% more frequent by the end of century
be. Te Fourth Assessment’s projections underscore the if emissions are not reduced. Te model produces more
dependence of sea levels upon greenhouse gas emissions years with extremely high areas burned, even compared
and the associated melt and ice-loss from Greenland and to the historically destructive wildfres of 20

17

and 2018.
Antarctica. If emissions continue at current rates, Fourth
Assessment model results indicate that total sea-level
rise by 2100 is expected to be 54 inches, almost twice the
rise that would occur if greenhouse gas emissions are
lowered to reduce risk.

1961-1990 2035-2064 2070-2099

This image shows the modeled area burned by wildfres from current time (modeled as 1961-1990), for mid-century (2035-2064), and for late
century (2070-2099). By the end of the century, California could experience wildfres that burn up to a maximum of 178% more acres per year
than current averages.

6

hile the impacts of climate change vary
over time and place, each community
will also experience these impacts
in unique ways that will depend on

social, economic, and demographic factors. The Fourth
Assessment makes new strides at the intersection of

W
Impacts of Climate Change on People

social and physical sciences to understand how climate
change will affect Californians – and how Californians
can adapt and safeguard their communities from climate
change.

PUBLIC HEALTH

Climate change will afect California’s diverse people
and communities diferently, depending on their
location and existing vulnerabilities. While research
shows that all Californians will likely endure more
illness and be at greater risk of early death because of
climate change, vulnerable populations that already
experience the greatest adverse health impacts will be
disproportionately afected.

Heat waves, the natural disaster responsible for the
most deaths in California over the last 30 years, are an
example of the current and future risk climate change
poses to people. Te 2006 heat wave killed over 600
people, resulted in 16,000 emergency department visits,
and led to nearly $5.4 billion in damages. Te human
cost of these events is already immense, but research
suggests that mortality risk for those 65 or older could
increase ten-fold by the 2090s because of climate change.
Studies show that while air conditioning can reduce
mortality and illness from heat, increased electrical
demand for cooling due to hotter conditions could
also drive up emissions. However, the state is rapidly
moving to cleaner electricity generation. Greenhouse gas
emissions from electricity generation in 20

16

were about
37% lower than emissions in 1990.

Social
Vulnerability
Composite
Score

A Fourth Assessment study produced this map of social vulnerability to heat by using

18

health, social, and environmental factors associated with heat vulnerability. The
map highlights the relative heat vulnerability of 8,046 census tracts by synthesizing
vulnerability indicators to render a clearer picture of overall heat vulnerability.
In more detail, Map A illustrates the Bay Area and Map B shows greater Los
Angeles area.

IMPACT FROM CLIMATE CHANGE: Heat-Health Events
(HHEs), which predict heat risk to local vulnerable populations,
will worsen drastically throughout the state by mid-century. The
Central Valley is projected to experience average HHEs that are
up to two weeks long, and HHEs could occur four to ten times
more often in the North Sierra region.

ACTION FOR RESILIENCE: The Fourth Assessment supported
the development of a prototype heat warning system known
as the California Heat Assessment Tool (CHAT), which was
designed to provide information about heat events most likely to
result in adverse health outcomes. It will support public health
departments taking action to reduce heat-related morbidity and
mortality outcomes.

7

new study found that deep greenhouse gas emission reductions (80% below 1990 levels) in California
could signifcantly improve health outcomes, and cost savings would be comparable to the cost of achieving
those reductions by 2050. These savings are achieved because shifting from polluting technologies to clean

technology improves air quality, saves lives, and improves overall public health.
A

In addition to heat, direct climate impacts like wildfre,
drought, and coastal and inland fooding will negatively
afect public health. However, there are also additional
indirect efects of climate change on human health:
wildfre smoke leads to increased respiratory illness,
warmer temperatures lead to the spread of mosquito-
borne diseases like Zika, and increased disasters lead to
greater stress and mental trauma.

CLIMATE JUSTICE

Te Fourth Assessment includes a report on climate
justice in California, a new addition to the assessment
process. Climate justice is the concept that no group of

people should disproportionately bear
the burden of climate impacts or the
costs of mitigation and adaptation, and
is a critical component of California’s
climate strategy.

Tis Fourth Assessment report
highlights the importance of adaptation
eforts to minimize climate impacts to
disadvantaged communities, as well
as case studies of innovative programs
to increase the resiliency of vulnerable
populations in California. Te report
identifes areas for additional research
needed to improve climate adaptation for

Vulnerable communities vulnerable populations and to promote
include feld workers, such climate justice in California. Tese
as this person being given include better tools, indices, maps, and
a protective N95 face mask metrics for identifying and quantifying
who was exposed to poor air resilience in vulnerable communities,
quality during the California research into achieving a just transition
wildfres in the fall/early to a low carbon economy, and methods
winter of 2017/2018. for ensuring community involvement in
Photo: CAUSE climate adaptation planning.

TRIBAL AND INDIGENOUS COMMUNITIES

For the frst time, the Fourth Assessment includes a
Tribal and Indigenous Communities Summary Report.
Tribes and Indigenous communities in California face
unique challenges under a changing climate. Tribes
maintain cultural lifeways and rely on traditional
resources (like salmon fsheries) for both social and

An example of how tribes use Traditional Ecological Knowledge can
be seen in the use of prescribed burns. These are commonly deployed
within a centuries-old cultural context to manage meadows, forests,
and other areas within tribal lands.

economic purposes. For many tribes in California,
seasonal movement and camps were a part of living
with the environment. Today, these nomadic options are
not available or are limited. Tis is the result of Euro-
American and U.S. policy and actions and underpins
several climate vulnerabilities. Tribes with reservations,
Rancherias, or allotments are vulnerable to climate
change in a specifc way: tribal lands are essentially
locked into fxed geographic locations and land status.
Only relatively few tribal members are still able to engage
in their cultural traditions as livelihoods.

Traditional Ecological Knowledge (TEK)-based methods
are gaining a revitalized position within a larger
statewide toolset to build resilience against climate
change by tribal and non-tribal stakeholders alike.
Te importance of maintaining TEK is not isolated to
environmental and ecological improvements. Tese
ancient, traditional practices are closely linked to
climate resilience across tribal cultural health, identity,
and continuity. Cultural practices and traditional land
management are also linked to improving physical
and mental health among tribal members. Tese TEK
techniques are increasingly incorporated by non-
tribal land and resource managers as part of wildfre
prevention and ecosystem management.

8

Impacts of Climate Change on Infrastructure

T he Fourth Assessment provides in-depth
assessments that support proactive steps to
protect California’s energy, transportation,
and water infrastructure systems and the

communities they serve. These systems face increasing
risks from climate change as temperatures warm, sea
levels rise, and other climate impacts worsen. These
systems are interconnected, and disruption in one part
can impact other connected parts with both direct and
indirect economic effects.

ENERGY

Energy resources can be considered from both supply
and demand perspectives. Fourth Assessment studies
found infrastructure that supplies energy along the coast
– particularly docks, terminals, and refneries – will
increasingly be exposed to coastal fooding. Meanwhile,
electrical power lines, rails, and roads are primarily
at risk from increasing wildfre. Costs and impacts of
wildfre to electricity transmission and distribution
systems are expected to grow as climate change impacts
increase.

IMPACT FROM CLIMATE CHANGE: Annual demand
for residential electricity is projected to increase in inland
and Southern California, with more moderate increases in
cool coastal areas. Increases in peak hourly demand during
the hot months of the year could be more pronounced.
Even though reduced use of natural gas in warmer winter
months will offset some of the total demand for energy, it
will be critical to be able to meet higher peak loads while
protecting infrastructure from climate impacts.

ACTION FOR RESILIENCE: Studies found that fexible
adaptation pathways that allow for implementation of
adaptation actions over time enable utilities to protect
services to customers most effectively. The California Public
Utilities Commission recently began a process to consider
strategies and guidance for climate adaptation for electric
and natural gas utilities, which will be informed by the
Fourth Assessment.

IMPACT FROM CLIMATE CHANGE: Emerging fndings for
California show that direct climate impact costs by the middle
of this century are dominated by human mortality, damages to
coastal properties, and the potential for droughts and damaging
foods. The costs have been estimated at tens of billions of
dollars. The impacts after the middle of this century will be
much lower if global greenhouse gas emissions are reduced
substantially.

ACTION FOR RESILIENCE: California’s Fourth Climate
Assessment contributes information and tools that are needed
from local to statewide levels to design and implement
adaptation measures to lower economic impacts. In addition, the
Climate-Safe Infrastructure Working Group, created in response
to Assembly Bill 2800 (Quirk), is releasing recommendations
that build on the Fourth Assessment fndings to inform a robust,
comprehensive, and equitable approach to building for the
future.

Solar panels produce energy at the California Department of Water
Resources Pearblossom Pumping Plant in Pearblossom, California. The
Fourth Assessment considered climate risk to the electricity system in
the context of the growth of renewable energy supply. Photo credit:
Florence Low/California Department of Water Resources 2017.

9

TRANSPORTATION

California’s roads, railroads, pipelines, waterways, ports,
and airports are critical for the movement of people
and goods. Tey will be signifcantly afected by climate
change. A growing threat to California’s transportation
system is wildfre, which can also have cascading efects
like landslides and mudslides that occur afer rain falls
on newly burned areas.

Increasing temperatures are also expected to increase
road construction costs between 3 and 9%. Adapting
roadway materials to withstand higher temperatures is
needed to avoid potential costs of over $1 billion by 2070.
1

15

miles of railroad could be at risk of coastal fooding
by 2040, with an additional 285 miles at risk by 2100.

The combination of the Thomas wildfre (281,893 acres) and a subsequent intense
rainstorm caused heavy mud and debris fows in the towns of Carpinteria and
Montecito, resulting in 21 fatalities, destroying at least 1,063 structures, causing
over $2.176 billion in damages, and closing Highway 101 for two weeks.

IMPACT FROM CLIMATE CHANGE: Miles of
highway at risk of fooding in a 100-year storm event
will triple from current levels to 370 miles by 2100.
Under that scenario, over 3,750 additional miles of
highway will be exposed to temporary fooding.

ACTION FOR RESILIENCE: Based in part on its
work with the Climate-Safe Infrastructure Working
Group, Caltrans will update its Highway Design
Manual to include the latest climate-informed data
on precipitation and heat. Caltrans will also complete
climate vulnerability assessments and develop climate
adaptation strategies for each of its 12 districts.

Airports in major urban areas including San Francisco
(SFO), Oakland, and San Diego will be susceptible to
major fooding from a combination of sea-level rise
and storm surge by 2040-2080, depending on location,
without implementation of protective measures. SFO is
already at risk of fooding from storm surge.

WATER INFRASTRUCTURE

Te impacts of climate change on California’s water
infrastructure and management are especially profound
and are causing shifs in the water cycle, greater risks to
engineered systems, and threats to ecosystems and water
quality. Te complex network that stores and distributes
water throughout the state was designed for historical
hydrologic conditions that are now changing. Te Fourth
Assessment contributes critical knowledge to understand
these new risks and to improve management.

Modeling of reservoir operations show that Shasta and
Oroville reservoirs, the two largest in the state, will have
roughly one-third less water stored annually by the end
of the century under current management practices.
Tis reduced storage could limit water supplies and
thus lower resilience to droughts. Changes in seasonal
precipitation combined with the efects of sea level rise
in the Delta may compound water supply reliability for

10

cities and farms that depend on imported water from
the State Water Project and Central Valley Project, as
exports from the Delta in future droughts could be
reduced by as much as 50% more than during historical
droughts. Te Fourth Assessment also found that water
rights administration and oversight practices from past
droughts are ill-suited to the growing challenges for
water management from climate change.

As temperatures increase, more precipitation will fall as
rain rather than snow. With potentially larger storms,
existing food management practices and infrastructure
will be challenged to meet the higher fows. Advances in
monitoring systems, forecasts, and coordination, coupled
with continuing modifcations and repairs to food
management infrastructure, will enable more time to
prepare for future large foods while increasing options
to improve and maintain supply reliability.

The Shasta Dam is one of California’s two largest, with a storage
capacity of 4.55 million acre feet. Photo credit: Apaliwal 2009.

IMPACT FROM CLIMATE CHANGE: Current management practices
for water supply and food management in California may need to be
revised for a changing climate. This is in part because such practices were
designed for historical climatic conditions, which are changing and will
continue to change during the rest of this century and beyond. As one
example, the reduction in the Sierra Nevada snowpack, which provides
natural water storage, has signifcant implications for California’s water
management system.

ACTION FOR RESILIENCE: Promising adaptation options such as the
use of probabilistic hydrological forecasts, better measurements of the
snowpack, and other improved ways to manage water can reduce these
negative impacts. Increased groundwater storage is another promising
option, which may include taking advantage of increased winter runoff
to food agricultural and natural areas to recharge aquifers. Institutional,
regulatory, legal, and other barriers may need to be overcome to
implement science-based solutions.

In addition to illuminating impacts from climate
change to California’s water infrastructure, the Fourth
Assessment also presents potential solutions from
around the state. One study shows how creative
approaches from local water districts better prepared
them for California’s drought. While small water systems
throughout the state currently struggle to incorporate
climate change into their planning and management
practices, the State could help disadvantaged
communities most impacted by climate change by
providing funding, technical assistance, and assistance
consolidating these water providers.

L and subsidence and sea-level rise will impede the function of levees in the
Sacramento-San Joaquin Delta, and by 2050-2080 some Delta levees may no
longer meet federal standards.

11

Impacts of Climate Change on Natural and Working Lands and Waters

N
atural and working lands and waters include
forests, rangelands, farmland, riparian areas,
and California’s ocean and coast. These lands
contribute to the natural infrastructure of

the state. They harbor the species and ecosystems of
California, and are increasingly at risk of disruption due
to climate change.

FORESTS

California’s forests cover almost one-third of the state
and provide important ecosystem services including
water capture and fltration, wildlife habitat, recreation
opportunities, and timber products. Climate change
poses increased risk of wildfre and potential for insect
infestations. California’s forests have the potential to
remove and store carbon from the atmosphere, and are
an important element of the State’s programs to reduce
carbon in the atmosphere. However, more research is
needed to understand the relationship between forest
management practices to reduce wildfre risk and the
efect on carbon storage. A Fourth Assessment study
found that fuel treatments lowered the biomass stored in
a forest, but that more of the remaining biomass survived
a fre than in an untreated forest area. Te study also
developed a new method to track how much biomass is
stored in living trees on large scales.

California’s forests contain over 60 species of trees including red fr (Abies
magnifca) and extends from coastal regions to high elevations in the Sierra Nevada
and other mountain ranges. Photo: Jean Pawek

12

ACTION FOR RESILIENCE: A Fourth Assessment
review of forest health literature provides further
scientifc backing to the State’s Forest Carbon Plan
to increase forest restoration and treatment such as
prescribed fre to an average of 35,000 acres a year
by 2020. Additionally, intensive thinning in highly
productive forests reduced tree evapotranspiration,
suggesting that forest thinning could result in
increased base fows of up to 10% for dry years and
5% for all years.

This review found prescribed fre a suitable tool to
lower extreme fre risk. However, under extreme fre
weather conditions, fres may simply jump or burn
through treated areas. With regards to sequestering
carbon, a key question in California forests is
whether fuel treatment data such as fre intensity,
stand age, and extent of treatment can be used to
predict the reduction of carbon lost in a subsequent
wildfre.

A Fourth Assessment wildfre model suggests a 77%
increase in mean and up to a 178% increase in maximum
area burned by wildfres (compared to 1961-1990) by
2050, but the actual impacts could be substantially more
severe because external factors such as wind are not yet
incorporated. By the end of the century, if greenhouse
gas emissions continue to rise, extreme wildfres burning
over about 25,000 acres is projected to increase by nearly
50%. Reducing tree density and restoring benefcial,
controlled fre can improve resilience of California’s
forests to wildfre. In the areas that have the highest
fre risk, wildfre insurance is estimated to rise by 18%
by 2055, and the fraction of property insured would
decrease.

RANGELANDS

Conservation of California’s grasslands, chaparral, and
oak woodlands and improved management of their
soils has strong potential to improve soil water-holding
capacity, increase stream fows and aquifer recharge,
reduce fooding and erosion, and reduce climate-related
water defcits. Increasing organic matter in soils by 3% by
applying compost could increase the soil’s water holding
capacity by up to 4.7 million acre-feet across all working
lands in California, with hydrologic benefts greatest in
locations with enough precipitation to fll increases in
soil storage capacity.

Many of California’s rangelands consist of nonnative grasses and
oak woodlands including these blue oaks (Quercus douglasii) or
chaparral. Photo: Neal Kramer

ACTION FOR RESILIENCE: Field experiments and modeling
show that a single application of compost to rangelands in
California can increase soil organic carbon sequestration for up
to 30 years and enhance net primary productivity. The resulting
increase in soil organic matter and increased vegetation also
supports infltration of water during storm events, contributing to
recharge of aquifers. A lifecycle assessment of California’s largest
organic waste streams — food waste, yard waste, and cattle
manure — showed that composting these feedstocks and applying
the compost to California rangelands has lower net greenhouse
gas emissions than other waste management approaches.

BIODIVERSITY AND HABITATS

California is a globally ranked biodiversity hotspot: only
25 regions in the world have as many species. Tese
species live in the state’s natural vegetation types: forests,
chaparral, riparian areas, riverside and wetlands, as well
as in its working landscapes, which include rangelands
and agricultural lands. Under current emissions levels,
between 45 to 56% of the natural vegetation in California
becomes climatically stressed by 2100. Te recent tree
die-of during the drought of 2012-2016 shows how
projected impacts are already having drastic efects.

Corridors can provide a means for plants and animals
to migrate to more suitable areas as the climate changes.
A Fourth Assessment study provides a framework for
climate-wise corridor design and implementation for
terrestrial plants and wildlife. It recommends starting
with designs based on land use and land cover, to
capture the connectivity needs of the majority of species.
Corridors should be prioritized that connect habitat
patches to sites where the future climate will be similar to
the current climate in the habitat patch and incorporate
climate refugia.

13

AGRICULTURE

California produces over half of the nation’s specialty
crops, including fruits, vegetables, nuts, fowers, and
nursery crops. Many of these crops, including fruit
and nut trees, are particularly vulnerable to climate
change impacts such as altered temperatures and stress
from warmth and dryness. Climate change impacts to
California agriculture will add to ongoing challenges
from conversion of agricultural land to urban areas and
regulatory challenges. California agriculture is projected
to experience lower crop yields due to extreme heat
waves, heat stress and increased water needs of crops
and livestock (particularly during dry and warm years),
and changes in pest and disease threats. Many of these
impacts can be lessened through on-farm management
practices, technological advances, and incorporation
of climate change risks in decision-making. A Fourth
Assessment study suggests that climate-related crop
losses will be less than impacts associated with the loss
of water supply and conversion of agricultural lands to
other uses.

An analysis of crops, dairies, and beef cattle in California
based on historical and projected climate conditions
suggests that agriculture will continue to thrive through
2050, although with a reduction of 5 to 15% in gross crop
revenues, assuming reductions in irrigation water. When
proper growing conditions exist, farms may rely on the
production of higher value crops to cope with rising
opportunity costs of water and land. Te high demand
for specialty crops means that production of these crops
will continue, while feld and grain crops may face more
important decreases in irrigated area and associated loss
of agricultural jobs.

California’s agriculture produces a high diversity of crops, and depends on water
that is frequently imported from other parts of the state or western US.
Photo:Patrick Huber

IMPACT FROM CLIMATE CHANGE: A
secondary, but large, effect of droughts is the
increased extraction of groundwater from aquifers
in the Central Valley, primarily for agricultural uses.
The pumping can lead to subsidence of ground
levels, which around the San Joaquin-Sacramento
Delta has been measured at over three-quarters of
an inch per year. This subsidence impacts the canals
that deliver water across the region.

ACTION FOR RESILIENCE: Flooding of some
types of agricultural felds during wet years can
provide some additional groundwater recharge,
which can be used to support agriculture through
longer droughts. This could be an important
adaptation option considering the loss of snowpack
forecasted for the rest of this century. California’s
Sustainable Groundwater Management Act will
also reduce groundwater overdraft, and guidance
for incorporating climate change projections will
increase resilience.

IMPACT FROM CLIMATE CHANGE: Agricultural
production could face climate-related water
shortages of up to 16% in certain regions.
Regardless of whether California receives more or
less annual precipitation in the future, the state will
be dryer because hotter conditions will increase the
loss of soil moisture.

ACTION FOR RESILIENCE: Increasing soil organic
matter by 3% by applying a ¼ inch of compost
could increase the soil water holding capacity by
up to 4.7 million acre-feet if applied to all working
lands in California.

Impacts of Climate Change on the Ocean and Coast

C alifornia’s iconic shoreline is integral to the state’s identity, but climate change is rapidly changing the
ocean and coast. The coastal region, which stretches over 1,200 miles of shoreline, is an economic
powerhouse that contributed $41.1 billion to the state’s GDP, provided $19.3 billion in wages and salaries,
and supplied 502,073 jobs in 2013. Rising sea levels, warming ocean waters, increasing acidity, and

decreasing dissolved oxygen levels will have effects that ripple far beyond the three-quarters of Californians who live
in coastal counties. The Fourth Assessment included a Coast and Ocean Summary Report for the frst time; this report
synthesizes the latest research – touched on below – about the challenges facing our coast and ocean because of
climate change and what actions we can take to increase their resilience.

OCEAN WARMING

California has recently experienced unprecedented
events along its coasts including a historic marine heat
wave, record harmful algal blooms, fsheries closures,
and a signifcant loss of northern kelp forests. Tese
events increase concern that coastal and marine
ecosystems are being transformed, degraded, or lost
due to climate change impacts, particularly sea-level
rise, ocean acidifcation, and warming. From 1900 to
2016, California’s coastal oceans warmed by 1.26 °F.
“Te Blob,” a very warm patch of ocean water of the
coast of California from 2013-2016, demonstrated that
anomalously warm ocean temperatures can produce
unprecedented events, including the mass abandonment
of sea lion pups and California’s record-setting drought.

IMPACT FROM CLIMATE CHANGE: A new model
estimates that, under mid to high sea-level rise scenarios, 31
to 67% of Southern California beaches may completely erode
by 2100 without large-scale human interventions. Damages
in the state’s major population areas could reach nearly $17.9
billion from inundation of residential and commercial buildings
under 20 inches of sea-level rise, which is close to the 95th
percentile of potential sea-level rise by the middle of this
century. A 100-year coastal food, on top of this level of sea-
level rise, would almost double the costs.

RISING SEA LEVELS

Building resilience
to sea-level rise in
California requires
approaches tailored to
communities’ needs,
climate impacts, and many
other factors. Options
to protect communities
and ecosystems include
combinations of armoring,
natural infrastructure,
and hybrid approaches.
Decision-makers
need tools to evaluate
the economic and
environmental costs and
benefts of alternative
strategies with more
complete information.
Te Fourth Assessment
contributed to this need

ACTION FOR RESILIENCE: A Fourth Assessment study
developed technical guidance on design and implementation
of natural infrastructure for adaptation to sea-level rise, such
as the use of vegetated dunes, marsh sills, and native oyster
reefs. This research included case studies on existing natural
shoreline infrastructure projects at fve sites spanning from
Humboldt to Los Angeles counties that show promising
approaches to increase resilience to sea-level rise and other
benefts.

The CoSMoS tool permits assessment of food
risk for all parts of California. This image shows
the San Diego Harbor with a 4.9 foot sea level
rise and with or without a 100-Year storm.

15

This site in Ventura County showed severe coastal erosion in 1990. A managed
retreat of infrastructure from the waterline provided adequate space for restoration
using cobble, sand, and dune plantings. To learn more about this project and other
case studies, see the brochure “Case Studies of Natural Shoreline Infrastructure in
Coastal California” that was prepared as part of the Fourth Assessment.

A Fourth Assessment study found that
sea-level rise has become the dominant
concern for coastal managers, and most

also face funding and fnancing barriers.

by supporting the expansion of CoSMoS – a tool that
can simulate sea-level rise in combination with storm
events and other coastal dynamics – to include Southern
California.

Coastal protection strategies can include the restoration
of tidal marshes, judiciously-placed coastal armoring,
and beach renourishment for highly accessed urban
locations (e.g., adding large volumes of sand, an
expensive solution lasting only 1-2 years). However, by
2050, with increasing sea-level rise and coastal storms,
localities may begin to consider retreat strategies.

Te restoration of marine plants and seaweeds in coastal
environments is a tactic that could increase dissolved
oxygen levels, at least for local areas. Ocean and coastal
vegetation including marshes also sequester carbon,
and quantifying the locations and contributions that
marine plants can make to reducing carbon dioxide in
local waters is needed. Other actions include reducing
nutrient runof from sewage disposal and excess
agricultural fertilizer.

OCEAN CONDITIONS

Te ocean has been absorbing atmospheric carbon
dioxide, which diminishes the amount of greenhouse
gases in the atmosphere and slows the rate of climate
warming but causes the ocean to become more acidic.
However, its capacity to do so will decrease. Improving
our understanding of the overlapping efects of rising

16

http:plantings.To

temperature, ocean acidifcation, and identifying
potential survival thresholds for species or ecosystems
will allow us to make better-informed decisions and
improve management options to reduce future losses and
impacts.

Ocean warming, ocean chemistry changes, sea-level rise,
and other greenhouse gas-driven changes to California’s
ocean and coast – those already occurring and projected
– will have signifcant consequences for California’s
coastal economy, communities, ecosystems, culture, and
heritage. Reducing greenhouse gas emissions is the most
efective long-term solution to man-made climate change
and ocean acidifcation.

Ocean-
Atmosphere
Influences

Changes
to Rainfall

Changes to
Ocean Currents

Ocean
THERMAL Temperature
Expansion

CO2 O2 Stratification

Acidity pH

Changes in Ocean Physiology/ Behavior/
Shell Formation Chemistry

IMPACT FROM CLIMATE CHANGE: Climate extremes and
ocean acidifcation are already impacting shellfsh in California.
Acidifcation affects shell-building species by decreasing the
carbonate ions available in the water that they need to build their
shells, causing larvae to essentially dissolve at certain acidities.

ACTION FOR RESILIENCE: A Fourth Assessment study found
a species of mussel can be an important “indicator species” for
California to help us understand the biological and chemical
processes altering ocean waters, potentially pointing the way
to strategies that are more effective for mitigating the harmful
effects of acidifcation.

Increased
Changes Greenhouse Gases
to Winds/
Storms/Waves

Foodweb
Effects

King Tides/
Storm Surge

• Coastal Communities
• Fisheries & Agriculture
• Human Health Sea-Level Rise
• Economic Growth
• Natural Heritage

COASTAL
Erosion

Climate change can affect many parts of the ocean ecosystem including what species can live in the ocean, foodwebs, winds and storms, ocean
currents, sea level rise, and ocean chemistry, particularly the acidity of the water and the level of dissolved oxygen held in the water.

17

Building Capacity to Address Local Impacts

F
or climate adaptation to be effective there is a
need for action from all levels of government.
Adaptation planning and actions at the
community level will need regional and local

context. The sector-specifc analyses and advanced
projections developed as part of the Fourth Assessment
are key to increasing resilience against natural disasters
and enabling effective local action.

EMERGENCY MANAGEMENT AND
DISASTER PREVENTION

Climate change is making major disasters more frequent
and destructive, and emergency managers are starting
to ensure their capacity matches growing challenges.
A Fourth Assessment study found that $1.7 billion of
critical facilities for emergency response, like dispatch
centers and fre stations, are at risk to wildfre or food
damage by 2100, and researchers developed a tool to
assess emergency infrastructure vulnerability.

IMPACT FROM CLIMATE CHANGE: In the City of
Los Angeles, eight days of power disruption due to a
prolonged heat wave would pose critical threats to lifeline
systems such as treated water, supplies, and access to air
conditioning.

ACTION FOR RESILIENCE: Integrated maps of
interconnected emergency services systems can help
make practitioners more aware of the importance of
cascading events and geographically-connected impacts
(teleconnections) and can support effective efforts to
prevent or otherwise mitigate them.

Another Fourth Assessment study shows that
interconnected systems are vulnerable to disasters in
ways that may be beyond the traditional jurisdictional
scope of local emergency managers. Maps of
interconnected lifeline systems will be needed to
recognize and prepare for cascading efects of climate
impacts.

Proactive planning for future urban growth will be
particularly important to avoid loss of life and property
in the future. Avoiding residential growth in areas at
high risk of wildfre and other forms of “climate-smart
development” will be critical to reducing vulnerability
to climate change. Future research is needed on the
interplay between climate risk and development patterns.

LOCAL AND REGIONAL GOVERNMENTS

In order to address the impacts of climate change,
California’s local and regional governments must
build institutional capacity to ensure the resilience
of individuals, communities, natural systems, and
infrastructure. Te Fourth Assessment explores the
social aspects of preparing people and communities
to grapple with and adapt to the imminent impacts of
climate change, particularly in light of the high cost of
natural disasters and other climate change-related events.

In addition to the social aspects of preparing
communities for the impacts of natural disasters and
recovery, local governments must identify strategies to
deal with the fnancial burden estimated to be in the
tens of billions of dollars. Given the potentially high cost
of inaction, climate adaptation is a highly cost-efective
option for governments to pursue.

A Fourth Assessment study found that
models that can quantify risks to people’s
assets can help engage stakeholders who

may be reluctant to participate in discussions
of climate vulnerability and adaptation by
allowing them to see how their communities
will experience the impacts of extreme climate-
related events.

While California’s three prior climate change assessments
were focused on developing climate models and
assessing climate change impacts, the Fourth Assessment
prioritized an additional focus: identifying actions for
successful climate change adaptation across diferent
sectors and regions.

18

IMPACT FROM CLIMATE CHANGE: A Fourth Assessment
study found that funding and fnancing challenges are
among the top barriers to adaptation, with these challenges
exacerbated by a number of organizational barriers such as
limited local government staff and lack of technical capacity,
agency leadership, and stakeholder partnerships.

ACTION FOR RESILIENCE: As part of the Fourth
Assessment, the Adaptation Capability Advancement Toolkit,
termed Adapt-CA, was created to help local governments
overcome common organizational barriers and advance their
capability to implement climate change adaptation measures.
The Toolkit can help local governments assess their existing

capabilities for climate adaptation and identify concrete
actions to advance their capabilities for more effective
planning and implementation of climate change adaptation
activities.

View the Adaptation Capability Advancement Toolkit
(Adapt-CA) at:

www.arccacalifornia.org/adapt-ca

The Alliance of Regional Collaboratives for Climate
Adaptation represents networks across California that
are building resilience to regional impacts. It hosts the
Adapt-CA Toolkit.

To support action at the local scale, the Fourth
Assessment includes reports for 9 regions of the
state. Tese summary reports were included for
the frst time as part of the State’s assessment
process in part because the vast majority of
adaptation planning and implementation will
happen at the local and regional scales. Each of
these regional reports provides a summary of
relevant climate impacts, adaptation solutions,
and local initiatives. As previously mentioned,
the Fourth Assessment also includes three
summary reports on climate justice, tribal and
indigenous communities, and the coast and
ocean. Like the regional summary reports,
each of these 3 reports was designed to catalyze
discussions, planning, and actions to understand
and address climate vulnerability.

Te map on this page shows the regions and the
icon for all 12 summary reports.

The Fourth Assessment produced nine regional reports and three topical
reports to provide greater detail for the public on the climate change risks and
potential adaptation strategies for California.

SAN
DIEGO

REGION

SAN FRANCISCO
BAY AREA
REGION

NORTH
COAST
REGION

CLIMATE
JUSTICE

OCEAN
AND COAST

COMMUNITIES

TRIBAL
COMMUNITIES

SACRAMENTO
VALLEY
REGION

SAN
JOAQUIN
VALLEY
REGION

SIERRA
NEVADA
REGION

LOS ANGELES
REGION

INLAND DESERTS
REGION

CENTRAL
COAST
REGION

These reports, the statewide summary report, 44 technical research reports, and other
resources are available on the Fourth Assessment website:

www.ClimateAssessment.ca.gov

19

http:www.ClimateAssessment.ca.gov

Acknowledgments

Multiple sources of funding, dozens of state agencies, and
hundreds of researchers from public universities, federal
agencies, and the private sector – not to mention a wide
range of stakeholders – made California’s Fourth Climate
Change Assessment possible. Tere is not enough space
here to list every person who contributed to the Fourth
Assessment, but additional acknowledgments can be
found at www.ClimateAssessment.ca.gov.

Thank you to everyone who contributed to
the Fourth Assessment!

STATE AGENCY
MANAGEMENT TEAM

Agency Leadership

Secretary John Laird
California Governor’s Offce
of Planning and Research
Chair Robert Weisenmiller
California Energy Commission
Director Ken Alex
California Governor’s Offce
of Planning and Research

Executive Management

Louise Bedsworth
California Strategic Growth
Council

Keali’i Bright
California Natural Resources
Agency
Drew Bohan
California Energy Commission

Management Staff

Jamie Anderson
California Department of
Water Resources

Pamela Doughman
California Energy Commission

Leah Fisher
California Governor’s Offce
of Planning and Research

Guido Franco
California Energy Commission

Nuin-Tara Key
California Governor’s Offce
of Planning and Research
Susan Wilhelm
California Energy Commission
Joseph Wraithwall
California Natural Resources
Agency

Editorial Board for the
Fourth Assessment

Dan Cayan
(Editor-in-Chief), Scripps
Institution of Oceanography,
University of California San
Diego

Susan Wilhelm
(Deputy Editor-in-Chief),
California Energy Commission

Associate Editors

Alan Sanstad
Lawrence Berkeley National
Laboratory

Fred Lipschultz
USGCRP

Glynis Lough
Union of Concerned Scientists

Guido Franco
California Energy Commission

Jamie Anderson
California Department of Water
Resources

James H. Thorne
University of California, Davis

John Andrew
California Department of Water
Resources

John Battles
University of California, Berkeley

Klaus Scott
California Air Resources Board

Michael Mastrandrea
Stanford University

Nuin-Tara Key
California Governor’s Offce of
Planning and Research

Pamela Doughman
California Energy Commission

Rupa Basu
California Offce of
Environmental Health Hazard
Assessment

Tamara Wall
Desert Research Institute

Terry Surles
California Institute for Energy
and Environment

Duane Waliser
NASA Jet Propulsion Laboratory

Julie Maldonado
University of California, Santa
Barbara

Louise Bedsworth
California Strategic Growth
Council

Robert Lempert
RAND, Head Reviewer of the
Peer Review Committee for the
Statewide Summary Report

Publication Team

Marketing by Design and
Della Gilleran

Technical Editor

Grayson Hough

Stakeholder Advisor

Bruce Riordan

SUGGESTED CITATION

Thorne, James H., Joseph
Wraithwall, Guido Franco.
2018. California’s Changing
Climate 2018. California’s
Fourth Climate Change
Assessment, California
Natural Resources Agency.

DISCLAIMER: This report summarizes the results of work sponsored by the California Natural Resources Agency and California Energy
Commission, in the context of broader scientifc literature. The information presented here does not necessarily represent the views of the
funding agencies or the State of California.

http:www.ClimateAssessment.ca.gov