More than a dozen military leaders say the impacts of climate change threaten military readiness and response and will increase instability and conflict around the globe.
Their assessments are included in a recent report, National Security and the Accelerating Risks of Climate Change, by the CNA Corporation’s Military Advisory Board. The report’s authors – including 16 retired generals and admirals from the Army, Navy, Air Force, and Marine Corps – conclude that climate change impacts will act as threat multipliers and catalysts. Projected warming, changes in precipitation, sea level rise, and extreme weather events will pose risks to security within the U.S. and abroad.
At home, some of the threats are here and now. Many of the nation’s military installations are in coastal areas vulnerable to rising sea levels and storm surges. For example, the low-lying Hampton Roads area of Virginia is home to 29 military facilities. Sea level in the area is projected to rise 1.5 feet over the next 20-50 years and as much as 7.5 feet by the end of the century. One advisory board member, Brig. Gen. Gerald Galloway, stressed that “unless these threats are identified and addressed, they have the potential to disrupt day-to-day military operations, limit our ability to use our training areas and ranges, and put our installations at risk in the face of extreme weather events.”
Figure 1: Sea level rise projections for the Hampton Roads region, which is home to 29 different military facilities. Source: CNA, 2014
In California, it’s almost impossible to avoid hearing about the drought. Restaurants serve water only upon request, “Save Our Water” radio ads run daily, and the issue headlines news broadcasts.
The persistent drought threatens to increase the risk of wildfires, damage crops, and harm wildlife. For example, UC Davis researchers estimate the state’s farm industry could lose $1.7 billion and nearly 15,000 jobs in 2014 due to the drought.
While Californians are no strangers to drought, this one in particular is cause for alarm. For the first time in the 15-year history of the National Drought Monitor, the entire state faces ‘severe’ (in yellow in Figure 1 below), ‘extreme’ (in red), or ‘exceptional’ (in dark red) levels of drought. In fact, October 2013-September 2014 could wind up being one of the driest periods in nearly 500 years.
Relief is unlikely to come soon. The Climate Prediction Center at the National Oceanic and Atmospheric Administration (NOAA) suggests the drought will persist and intensify in California through the summer.
Figure 1. The entire state of California is experiencing severe, extreme, or exceptional drought. This is the first time this has happened since the Drought Monitor began such classifications 15 years ago. Source: US Drought Monitor: California, (National Drought Mitigation Center, 2014), http://droughtmonitor.unl.edu/Home/StateDroughtMonitor.aspx?CA
National Climate Assessment
People across the United States are dealing with the impacts of climate change: Farmers and ranchers across the Great Plains battle drought, transportation planners consider how floods might affect roads and bridges, and utility managers try to keep the electricity flowing during heat waves.
The Third National Climate Assessment (NCA), released May 6, 2014, is a compendium of the ways climate change affects our lives, livelihoods, and the economy in general. It describes how the climate has changed over the past century and provides a glimpse of future climate change and its impacts. The report also looks at how society is responding to a changing climate, and identifies actions we can take to prepare.
What is the NCA?
The NCA is a congressionally mandated report to the president and Congress that “analyzes the effects of global change on the natural environment, agriculture, energy production and use, land and water resources, transportation, human health and welfare, human social systems, and biological diversity.”
It’s a comprehensive synthesis describing how the climate has changed in different regions of the country, and the impacts to these regions and various sectors that scientists expect in the 21st century.
The report advances the concept of climate change as a “risk management” challenge, laying out the critical services, like access to water and energy, and natural resources likely to be disrupted or harmed by climate impacts.
The NCA can be a useful tool for public and private sector decision-makers trying to address climate challenges.
What are the key findings?
The report tells us that:
- Climate change is not just a problem for the future; it is happening here and now and is occurring faster than previously expected.
- Changes in some U.S. weather extremes, like heat waves and heavy rainfall events, are related to human-induced climate change.
- Climate impacts are threatening Americans’ health, infrastructure, water supplies, agriculture, ecosystems, and oceans.
- The costs of climate change are high and will increase if emissions are not controlled.
- Americans have tools at their disposal and are already beginning to take steps to reduce emissions, develop clean energy, and build resilient cities and industries that can withstand the effects of extreme weather.
The report identifies opportunities for adaptation, advocating that actions taken now are likely to return greater benefits than actions taken in the future. In addition, the NCA notes that efforts to reduce greenhouse gas emissions are critical for limiting the amount and rate of future climate change, giving adaptation investments a better chance of success.
How can I learn more?
The report is divided into several large sections. We provide links to these portions of the report and a short description of each.
Overview and Report Findings: Summarizes the assessment and the 12 top-line messages of the report.
Our Changing Climate: Explores observed and projected climate change at the global and national scale. Discussion focuses on physical changes, such as temperature, precipitation, and sea level. Observations tend to cover the last 50 to 100 years, while projections extend through the 21st century. Many maps and graphs in this section are referenced throughout the report.
Regions: Discusses in depth the observed and projected impacts for specific U.S. regions.
- Southeast and Caribbean
- Great Plains
- Hawaii and U.S. Pacific Islands
Sectors: Explores how climate affects economic, social, or ecological resources, typically cutting across geographic boundaries.
- Human Health
Cross-cutting sectors: Investigates how some types of communities are affected by climate impacts, the interrelationships among many resource decisions, and the large-scale changes in biogeochemistry that accompany climate change. Many of these chapters were not in previous assessments.
- Urban infrastructure and vulnerability
- Indigenous peoples, lands, and resources
- Rural communities
- Energy, water, and land use
- Land use and land cover
- Biogeochemical cycles
Responses: Looks at solutions, as well as the research, data, and tools that will facilitate the implementation of solutions.
- Decision Support
- Research Needs
- Sustained Assessment
How is the report assembled?
The NCA was written by 240 authors with diverse expertise and experience. They include academic researchers; local, state, and federal government officials; private sector leaders; and non-profit experts. Their efforts are coordinated by the U.S. Global Change Research Program (USGCRP), a collaboration of 13 federal science agencies. A 60-member federal advisory committee, the National Climate Assessment Development Advisory Committee (NCADAC), oversees the development of the NCA report and makes recommendations about the ongoing assessment process.
Previous assessments were released in 2000 and 2009. The latest update is expected to be the start of an ongoing process in collecting and disseminating data and information through various digital media and user networks.
What is the difference between the NCA and IPCC reports?
The NCA is independent of reports issued by the Intergovernmental Panel on Climate Change (IPCC). Although both are based on the latest science, and reach similar conclusions, they differ significantly their scope and focus. Here are some key differences:
- The NCA discusses observed and projected climate impacts in the United States, focusing on eight sub-regions, while the IPCC discusses observed and projected climate impacts at the continental scale.
- The NCA is subject to review by the National Academy of Sciences, government agencies, and the public. The IPCC is subject to scientific and governmental review.
Heavy Precipitation and Climate Change
Extreme precipitation events have produced more rain (Figure 1) and become more common (Figure 2) since the 1950s in many regions around the world, including much of the United States. In particular, the Midwest and Northeast have exhibited the strongest increases in the amount of rain falling in heavy precipitation events.
Scientists expect these trends to continue as the planet continues to warm. Warmer air can hold more water vapor. For each degree of warming, the air’s capacity for water vapor goes up by about 7 percent. An atmosphere with more moisture can produce more intense precipitation events, which is exactly what has been observed, averaged over large areas of the Earth.
It is important to note that increases in heavy precipitation may not always lead to an increase in total precipitation over a season or over the year. Some climate models project a decrease in moderate rainfall, and an increase in the length of dry periods, which offsets the increased precipitation falling during heavy events.
Figure 1: The map shows percent increases in the amount of precipitation falling in very heavy events (defined as the top 1% of all daily events) from 1958 to 2011 for each region.
Source: National Climate Assessment
Threats posed by Heavy Precipitation
The most immediate impact of heavy precipitation is the prospect of flooding as streams and rivers in the region overflow their banks. Since 2008, the United States has seen six floods costing at least $1 billion each, resulting in damaged property and infrastructure, agricultural losses, displaced families, and loss of life.
- In September 2013, Boulder, Colorado, received almost a year’s worth of rainfall (17 inches) in four days. The resulting flooding destroyed homes, shut down thousands of oil and gas wells, and damaged crops.
- In 2010, almost 20 inches of rain fell on Nashville, Tennessee, over three days. Losses in Nashville alone totaled over $1 billion.
- In 2008, floods struck the Midwest, with the worst impacts in Iowa and Wisconsin. Losses totaled $15 billion, mainly from property and agriculture.
In addition to flooding, heavy precipitation also increases the risk of landslides. When above-normal precipitation raises the water table and saturates the ground, slopes can lose their stability, causing a landslide. A particularly deadly landslide occurred in March 2014 in Washington state, where landslide risks can be relatively high. The heavy precipitation in the preceding weeks caused a landslide that buried 30 homes and killed at least 41 people
Excessive precipitation can also degrade water quality, harming human health and the ecosystem.. Storm water runoff, which often includes pollutants like heavy metals, pesticides, nitrogen, and phosphorus, can end up in lakes, streams, and bays, damaging aquatic ecosystems and lowering water quality for human uses. In the Chesapeake Bay, elevated levels of nutrients such as nitrogen and phosphorus have led to algae outbreaks, which can lower the water oxygen content, killing clams, oysters and other aquatic life.
Many cities in the United States, such as New York and Philadelphia, use a combined sewer system, where both storm water and wastewater are mixed, treated, and released. Heavy precipitation events can overwhelm such systems, sending excess storm water and wastewater directly into the environment. In the aftermath of Hurricane Sandy, overwhelmed sewer systems discharged 3.45 billion gallons of untreated sewage into New York City’s rivers, bays and canals. Even in Washington D.C., where coastal flooding was not a factor, 475 million gallons of untreated sewage was discharged into local rivers, drastically reducing water quality.
Figure 2: Relative frequency of once-in-five-year precipitation events falling over a 2-day period, averaged over the continental U.S, 1900-2011. Green bars in the recent decades show that these events have become more frequent, when averaged across much of the country.
Source: National Climate Assessment
How to Build Resilience
Communities can bolster their resilience and reduce the impacts of heavy precipitation by:
- Locating buildings and infrastructure on higher ground or areas that are less prone to flooding, raising buildings, or using flood control infrastructure.
- Limiting the use of non-permeable surfaces like pavement and concrete in developed areas, or replacing pavement with “green infrastructure” that can reduce runoff during storms.
- Separating storm water systems from wastewater systems, using holding ponds, or increasing water treatment capacity to avoid sending untreated sewage into local waterways.
Purchasing flood insurance can help families and communities recover after a flood hits. However, recent floods have put the National Flood Insurance Program billions in debt, and further reforms are necessary if public flood insurance will continue to be available in the future.
To Learn More
Wildfires and Climate Change
The number of large wildfires has nearly doubled since the 1980s, and the average length of wildfire season has grown by more than two months.
Research shows that changes in climate, especially earlier snowmelt and warming in the spring and summer, have helped boost this increase in fire activity in parts of the West. For much of the West, projections show that an average annual temperature increase of 1 °C would increase the median burned area per year. The increase could be as much as 600 percent in some types of forests
Wildfire risk depends on a number of factors, including temperature, soil moisture, and the presence trees, shrubs and other potential fuel. All these factors have strong direct or indirect ties to climate variability and climate change. Warmer temperatures and drier conditions can often help increase the chances of a fire starting, or help a burning fire spread. Such conditions also contribute to the spread of the mountain pine beetle and other insects that can weaken or kill trees, building up the fuels in a forest. Although our choices regarding land use and firefighting tactics can also play a role in lowering or raising risks, observed and anticipated changes in climate have and are expected to increase the area affected by wildfires in the United States.
Threats Posed by Wildfires
Since 2000, nine forest fires in the U.S. have caused at least $1 billion in damages each, mainly from the loss of homes and infrastructure, along with firefighting costs.
- In 2011, the Las Conchas Fire in New Mexico became the state’s largest in history by a factor of three. In 2012, that record was broken as the Whitewater-Baldy Complex fire burned nearly twice as many acres as the Las Conchas Fire.
- Wildfires burned more than 9 million acres in 2012. Colorado’s two most destructive fires ever—the Waldo Canyon and High Park fires -- happened during this particularly destructive season.
- From 2007-2008 more than 4,000 homes were destroyed in California alone. More than 15 million acres total burned across the southwest during these two exceptional fire years.
Estimates of the percentage increase in the area burned in regions across the West for a 1.8 degrees Fahrenheit, 1 degree Celsius warming. The different regions correspond to different “ecoprovinces,” which distinguish areas with distinctive vegetation types and climate conditions. Values are drawn from median burn estimates from a fire model. All areas exhibit increases; many of them exceed a doubling (i.e., value shown is more than 100%) and some areas show a five-fold increase (i.e., value shown is more than 400%). Source: National Research Council, "Climate Stabilization Targets: Emissions, Concentrations, and Impacts over Decades to Millennia ( 2011 ).
Damage to homes and other buildings can be substantial, in part from the recent and rapid development of areas near fire-prone forests. As the number of homes located near forests at risk of wildfire has increased over the past two decades, U.S. Forest Service fire suppression expenditures more than doubled between 1991-2000 and 2001-2010. State wildfire expenditures also increased substantially. While more buildings add to the risk of damage from natural fires, the presence of people in wildlands increases the risk of fires starting. From 2001 through 2011, 85 percent of wildfires were started by people.
Beyond direct damage to the landscape, several public health risks are related to wildfires. Smoke reduces air quality and can cause eye and respiratory illness. Wildfires can also hasten ecosystem changes and release large amounts of CO2 into the atmosphere—contributing to further climate change.
How to Build Resilience
Communities, builders, homeowners and forest managers can reduce the likelihood and impacts of wildfires by:
- Discouraging residential developments near fire-prone forests through smart zoning rules.
- Increasing the e space between structures and nearby trees and brush, and clearing space between neighboring houses.
- Incorporating fire-resistant design features and materials in buildings.
- Increasing resources allocated to firefighting and fire prevention.
- Removing fuels, such as dead trees, from forests that are at risk.
- Developing recovery plans before a fire hits, and implementing plans quickly after a fire has occurred to reduce erosion, limit flooding, and minimize habitat damage.
To Learn More
Water for Energy and Energy for Water: Challenges and Opportunities for Utilities
A series of three webinars sponsored by the Association for Metropolitan Water Agencies, the Water Information Sharing and Analysis Center, and the Center for Climate and Energy Solutions intended to help utility managers address issues across the water-energy nexus.
- Webinar 1 – An overview of water/energy issues from national and federal perspectives
May 8, 2 p.m. – 3 p.m. ET
Dr. Craig Zamuda from the Department of Energy (DOE) presents key findings from DOE’s recently released water/energy nexus report, attempting to distill some of the key issues and risks of which water and electric utilities should be aware. Dr. Kristen Averyt, Associate Director for Science for the Cooperative Institute for Research in Environmental Sciences and Director of the Western Water Assessment at the University of Colorado, presents her research regarding water-energy challenges that exist currently and are on the horizon.
- Webinar 2 - Partnerships between water and energy utilities to address water/energy challenges
June 19, 2 p.m. – 3 p.m. ET
Patrick (Pat) Davis, Sustainability Manager at Orange Water and Sewer Authority (Carrboro, North Carolina) and Doris Cooksey, Water Quality & Planning Manager at CPS Energy (San Antonio, Texas) share their experiences in addressing water-energy issues, focusing on how they have coordinated and developed shared strategies with peer utilities located in the communities that they serve.
- Webinar 3: Innovation and effective stakeholder engagement on water and energy issues
July 24, 2 p.m. – 3 p.m. EDT
Involving other stakeholders or partners for a water-energy project often leads to insights, innovations, and/or greater efficiency. In this third and final webinar, speakers from American Water and East Bay Municipal Utility District (EBMUD; California) discuss how they leveraged stakeholder involvement to address water-energy challenges and implement innovations.
Suzanne Chiavari, Engineering Practice Leader from American Water, will describe some of her organization’s recent work in using renewable energy technologies, and how they’ve engaged community partners to establish greater integration across their resource management activities. Clifford Chan, Manager of Water Treatment and Distribution at EBMUD, will talk about two projects with multiple stakeholders that have helped the utility to implement its energy management strategy.
Webinar 1: An overview of water/energy issues from national and federal perspectives
May 8, 2014
2 p.m. – 3 p.m. ET
Dr. Craig Zamuda from the Department of Energy (DOE) will present key findings from DOE’s recently released water/energy nexus report, attempting to distill some of the key issues and risks of which water and electric utilities should be aware. Dr. Kristen Averyt, Associate Director for Science for the Cooperative Institute for Research in Environmental Sciences and Director of the Western Water Assessment at the University of Colorado, will present her research regarding water-energy challenges that exist currently and are on the horizon.
Hurricanes and Climate Change
Definition of a Hurricane
A hurricane is a type of tropical cyclone, which is a general term for a low-pressure system with activity like thunder and lightning that develops in the tropics or subtropics, between about 30 degrees north and 30 degrees south latitude. In the Northern Hemisphere, these storms rotate counter-clockwise. In the Southern Hemisphere, they rotate clockwise. Stronger systems are called “hurricanes” or “typhoons,” depending on where they form. Weaker tropical cyclones might be called “tropical depressions” or “tropical storms.”
It’s unclear whether climate change will increase or decrease the number of hurricanes, but warmer ocean surface temperatures and higher sea levels are expected to intensify their impacts.
Hurricanes are subject to various climate change-related influences. Warmer sea surface temperatures could intensify tropical storms wind speeds, potentially delivering more damage if they make landfall. Based on sophisticated computer modeling, scientists expect a 2-11 percent increase in average maximum wind speed, with more occurrences of the most intense storms. Rainfall rates during these storms are also projected to increase by about 20 percent.
In addition, sea level rise is likely to make future coastal storms, including hurricanes, more damaging. Globally averaged, sea level is expected to rise by 1-4 feet during the next century, which will amplify coastal storm surge. For example, sea level rise intensified the impact of Hurricane Sandy, which caused an estimated $65 billion in damages in New York, New Jersey, and Connecticut in 2012, and much of this damage was related to coastal flooding.
The connection between climate change and hurricane frequency is less straightforward.
Globally, the number of tropical storms that form each year ranges between 70 and 110, with about 40 to 60 of these storms reaching hurricane strength. But records show large year-to-year changes in the number and intensity of these storms.
It’s important to note that changes in frequency and intensity vary from basin to basin. In the North Atlantic Basin, the long-term (1966-2009) average number of tropical storms is about 11 annually, with about six becoming hurricanes. More recently (2000-2013), the average is about 16 tropical storms per year, including about eight hurricanes. This increase in frequency is correlated with the rise in North Atlantic sea surface temperatures, which could be partially related to global warming.
One trend analysis published in the journal Nature shows the strongest hurricanes have also increased in intensity over the past two or three decades in the North Atlantic and Indian Oceans. Other areas in the Pacific and Indian Oceans show virtually no significant trends. Other trend analyses that include all hurricanes globally are similarly inconclusive, with upward trends in the North Atlantic and Indian Oceans and no apparent increase in frequency or intensity in other basins.
For the 21st century, some models project no change or a small reduction in the frequency of hurricanes, while others show an increase in frequency. More recent work shows that there is a trade-off between intensity and frequency – that as warmer oceans bolster hurricane intensity, fewer storms actually form. For the continental United States in the Atlantic Basin, models project a 75 percent increase in the frequency of category 4 and 5 hurricanes despite a possible decrease in the total frequency of all storms.
The tracks of tropical storms near the United States. Tropical storms are shown with red lines and major hurricanes (Category 3 and higher) are yellow. Source: National Hurricane Center. To see the legend, click here
Threats Posed by Hurricanes
The National Hurricane Center categorizes Atlantic hurricanes based on wind speed. A storm with winds exceeding 74 mph is a Category 1 hurricane. Storms with winds stronger than 111 mph are considered “major hurricanes” (Category 3 or higher). Many factors contribute to a hurricane’s impact, including its track, size, storm structure, rainfall amount, duration, and the vulnerability of the area it affects.
Eight of the 10 costliest hurricanes on record in the United States have occurred since 2004. Hurricanes Katrina (2005) and Sandy (2012) were by far the most damaging, costing $125 billion and $65 billion respectively. Hurricanes Andrew (1992) and Ike (2008) cost $27 billion each. Sandy ranks as the second most damaging storm or weather disaster since 1980, even though the storm was no longer a hurricane at landfall.
An important driver of the increased cost of hurricanes is increasing development in coastal areas. U.S. coastal populations grew by nearly 35 million people between 1970 and 2010. Coastal counties account for nearly 40 percent of the total U.S. population. As more development occurs in harm’s way — regardless of climate change — the more likely the damage will grow.
How to Build Resilience
Ways communities can bolster their resilience and reduce the impacts of hurricanes include:
- Preserving coastal wetlands and dunes to absorb storm surges.
- Replenishing beaches and improve infrastructure that affords coastal protection, such as seawalls.
- Elevating vulnerable buildings to reduce flood damage.
- Designing structures to be resilient to high winds and flying debris.
- Enacting policies that discourage development in vulnerable areas.
- Preparing prior to a storm’s arrival by boarding windows, clearing property of potential flying debris, and having an evacuation plan.
To Learn More
Extreme Heat and Climate Change
Across the globe, warming temperatures are increasing the risk of hot weather and decreasing the risk of colder weather. Hot summer days are getting hotter, as well as more frequent, sometimes stretching into multiday heat waves.
During the past decade, daily record high temperatures have occurred twice as often as record lows across the continental United States, up from a near 1:1 ratio in the 1950s. By midcentury, if greenhouse gas emissions are not significantly curtailed, scientists expect 20 record highs for every record low. The ratio could be 50:1 by the end of the century. By the 2050s, many of the Mid-Atlantic States including urban parts of Maryland and Delaware could see a doubling of days per year above 95 degrees F. In parts of the South, the frequency of days above 95 degrees F could triple, to over 75 days per year. While climate change will not mean the end of cold weather — scientists still expect record lows to occur in 2100 — the odds have clearly shifted (see map).
Extreme heat can also increase the risk of other types of disasters. When heat occurs in conjunction with a lack of rain, drought can occur. This, in turn, can encourage more extreme heat, as the sun’s energy acts to heat the air and land surface, rather than to evaporate water. Hot, dry conditions also increase the risk of wildfires, like the ones in 2013 in Colorado that were fueled by record heat and an ongoing drought.
More extreme heat in the 21st century. The map shows the projected increase in the number of days with maximum temperatures above 95°F for the later part of the 21st century. The color indicates the increase in the number of days (i.e., orange areas in Oklahoma are projected to experience 20 or more additional days of temperatures above 95°F). The hatching across most of the country indicates that the change passes a significance test in that location. Source: NOAA Technical Report NESDIS 142-9
Threats Posed by Extreme Heat
Extreme heat is the most deadly natural disaster in the U.S., killing on average more people (about 600 per year) than hurricanes, lightning, tornadoes, earthquakes, and floods combined. The Billion Dollar Weather Disasters database compiled by the National Oceanic and Atmospheric Administration lists heat waves as four of the top 10 deadliest U.S. disasters since 1980. Two heat waves in 1980 and 1988 were particularly deadly, contributing to 10,000 and 7,500 deaths respectively. These two events account for the vast majority of extreme weather-related deaths in the database, with Hurricane Katrina (1,833 deaths) as the only non-heat wave event that caused more than 500 fatalities.
High humidity and elevated nighttime temperatures (i.e., when nighttime low temperatures remain relatively warm) appear to be key ingredients in causing heat-related illness and mortality. Heat stress occurs in humans when the body is unable to cool itself effectively. Normally, the body can cool itself through sweating, but when humidity is high, sweat will not evaporate as quickly, potentially leading to heat stroke. When there’s no break from the heat at night, it can cause discomfort and lead to health problems, especially for the poor and elderly.
High temperatures at night can be particularly damaging to agriculture. Some crops require cool night temperatures, and heat stress for livestock rises when animals are unable to cool off at night. Heat-stressed cattle can experience declines in milk production, slower growth, and reduced conception rates.
Higher summer temperatures will increase electricity use, especially during heat waves. An increase in cooling demand is already apparent over the past 20 years. Although warmer winters will reduce the need for heating, modeling suggests that total U.S. energy use will increase in a warmer future. In addition, as rivers and lakes warm, their capacity for absorbing waste heat from power plants declines. This can reduce the thermal efficiency of power production, make it difficult for power plants to comply with environmental regulations regarding their cooling water, and can make it harder to get permits for new facilities.
How to Build Resilience
Communities can bolster their resilience and reduce the impacts of extreme heat by:
- Creating heat wave preparedness plans, identify vulnerable populations, and open cooling centers during extreme heat.
- Using green roofs, improved building materials, and shaded building construction to reduce the urban heat island effect.
- Pursuing energy efficiency to reduce demand on the electricity grid, especially during heat waves.
- Shading and cooling livestock, breeding livestock selectively for heat tolerance, and switching to growing more heat-resistant crops.