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.
To Learn More
A lot of folks in the eastern half of the United States are breathing a sigh of relief that spring is just around the corner. Average temperatures this winter were among the Top 10 coldest in some parts of the Upper Midwest and South. More than 90 percent of the surface of the Great Lakes is frozen, the highest in 35 years.
But while East Coast and Midwest kids have been sledding and their parents have been shoveling, it has not been cold everywhere. In fact, many areas are unusually warm.
In Alaska, January temperatures were as high as they have been in 30 years. The Iditarod dogsled race was especially treacherous this month because of a lack of snow. Crews had to stockpile and dump snow on the ground at the finish line in Nome, where temperatures earlier this winter broke a record.
Globally, January was the fourth warmest on record – really – despite pockets of well-below-normal temperatures in parts of the United States. According to the National Oceanic and Atmospheric Administration (NOAA), most areas of the world experienced warmer-than-average monthly temperatures. For example:
- China experienced its second warmest January on record.
- France tied its warmest January.
- Parts of Brazil and Australia saw record heat.
January temperatures were above normal for much of the globe.
Most people at some point develop a “Plan B” – in case their first choice of college doesn’t accept them, or it rains on the day of their planned outdoor party, or the deal for the house they wanted falls apart. The same principle applies for more dire situations, such as a city having plans in hand for an orderly evacuation in case of a large-scale disaster. We hope such an event will never happen, but the mayor had better be prepared in case it does.
In a commentary today in the scientific journal Nature Climate Change, three colleagues and I discuss the need for a “Plan B” for climate change: How will we cope with increasingly severe climate impacts if we are unsuccessful in limiting global warming to a chosen target?
In the 2009 Copenhagen climate accord, countries set a goal of limiting global warming to below 2 °C (3.6 °F) above the average global temperature of pre-industrial times. However, given that the planet has already warmed by 0.8 °C, additional warming is already locked into the system, and global greenhouse gas emissions continue to rise, this “Plan A” has become increasingly difficult and may become impossible to achieve if widespread emissions reductions do not begin within this decade. A maximum warming target is a necessary goal of climate policy, but what if our efforts fall short?
Some voices in the environmental community will feel that asking this question is ceding failure, but I disagree. Instead, it means admitting that we can’t perfectly foresee the future and that we need to be prepared for surprises. This is called risk management and everyone from parents, to mayors, to companies, to the U.S. military uses risk management every day to cope with uncertainty.
As President Barack Obama prepares to deliver his State of the Union address, we believe it’s a good time to take a look at the state of our climate: the growing impacts of climate change, recent progress in reducing U.S. emissions, and further steps we can take to protect the climate and ourselves.
The consequences of rising emissions are serious. The U.S. average temperature has increased by about 1.5°F since 1895 with 80 percent of this increase occurring since 1980, according to the draft National Climate Assessment. Greenhouse gases could raise temperatures 2° to 4°F in most areas of the United States over the next few decades, bringing significant changes to local climates and ecosystems.
This week’s brief but bitter cold snap over more than half the country prompted intense discussion about the polar vortex ranging from educational to bombastic.
|Figure 1: A depiction of the “average” polar vortex on Jan. 6. The winds of the vortex correspond to the narrow “rainbow” areas. The map is an average of the upper atmosphere’s “topography” (specifically, the 500 millibar height) from all the January 6ths between 1980 and 2010.|
|Figure 2: The polar vortex on Jan. 6, 2014. The ridge (“R”) and trough (“T”) responsible for relatively warm weather in much of the West and bitterly cold weather in the Midwest and East have been labeled.|
So let’s be clear: The cold snap this week was unusual but not entirely unprecedented. A few super-cold days don’t disprove global warming, just like a day of rain doesn’t end a drought. At the same time, we don’t yet know whether climate change will change the odds of future outbreaks of bitter cold. Research is still underway, and as of now, we shouldn’t necessarily expect these events to be more or less frequent in future winters.
Here’s a Q&A to cut through the hype:
- What is the polar vortex? The polar vortex describes the air circulating aloft (thousands of feet above the ground) about the North Pole, and its extent is marked by a ribbon of strong winds that is often called the “jet stream.” (We most commonly focus on the North Pole, but a similar circulation is present around the South Pole, too).
In the map (Figure 1), which is from the point of view of the North Pole, the vortex corresponds to purple and blue colored areas. The band where the colors change from blue/purple to red/yellow indicates the location of the jet stream, or the outer edge of the vortex. Winds are strongest where this color gradient is tightly packed (e.g., over the Pacific Ocean and North Atlantic Ocean). It tends to be quite cold at the surface below the purple areas, and warmer under the red/yellow areas.
It’s important to note that this figure is an average of many winter days. On any given day, we would see a number of deviations from this average pattern.
- What happened this week? Comparing this week (Figure 2) to the average picture (Figure 1), we can see that the purple area of the vortex has contorted and moved farther south. Along with this pattern, there are substantial “wiggles” in the jet stream. These deviations in the circulation helped bring cold air into the continental United States that normally stays in northern Canada and the Arctic. Meteorologists look for these wiggles, called “ridges” and “troughs” (“R” and “T” on the map) when putting together a forecast. While the trough brought notable cold to the Midwest and the East, the ridge has kept parts of the West warmer than average and relatively dry (much to the dismay of skiers).
Joe Casola, staff scientist and program director of Science and Impacts, and Dan Huber, science and policy fellow, co-authored this article.
The terms “climate change” and “global warming” might conjure up images of balmy beaches and scalding deserts – a world without winter. But it’s more complicated than that.
As we prepare for the official arrival of the season on Dec. 21, let’s look at a few ways winters in the United States are changing because of global warming, and what we can do to adapt.
A year after Hurricane Sandy, more work remains to be done to help families and communities fully recover. But another pressing need, not only for those who were in Sandy’s wake but for all of us, is to learn from the storm’s devastating impacts and reduce the risk of future damage and loss of life.
Hurricane Sandy's estimated $65 billion in damages make it the second costliest hurricane in U.S. history, surpassed only by Hurricane Katrina.
Building resilience to the impacts of major coastal storms like Sandy—and to other types of extreme weather that are becoming more intense and frequent as a result of climate change—will require a commitment to better protect infrastructure and implement policies to help get people out of harm’s way. Both efforts should take into account how future sea level rise can amplify storm surges, potentially making future impacts greater than what we’ve experienced in the past.
When I founded a new nonprofit organization 15 years ago, the United States and the world urgently needed practical solutions to our energy and climate challenges. That need has only grown more urgent.
Earlier today, I announced my plans to step aside as the President of the Center for Climate and Energy Solutions (C2ES) once my successor is on board. As I look back, I find we have come a long way. That said, any honest assessment of our progress to date in addressing one of this century’s paramount challenges must conclude that we have much, much further to go.
When our organization, then named the Pew Center for Global Climate Change, first launched in 1998, 63 percent of the world’s electricity generation came from fossil fuels. Incredibly, that number is even higher today – 67 percent. The concentration of carbon dioxide in the atmosphere, the main driver of climate change, is also higher than it was then – in fact, at its highest level in more than 2 million years.
Scientists around the globe have just reaffirmed with greater certainty than ever that human activity is warming the planet and threatening to irreversibly alter our climate. Climate change is no longer a future possibility. It is a here-and-now reality. It’s leading to more frequent and intense heat waves, higher sea levels, and more severe droughts, wildfires, and downpours.
We at C2ES have believed from the start that the most effective, efficient way to reduce greenhouse gas emissions and spur the innovation needed to achieve a low-carbon economy is to put a price on carbon. It’s a path that a growing number of countries, states, and even cities are taking.
The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) brings policymakers and the public up to date on the state of climate science. The IPCC report, released in stages, is the most comprehensive assessment of existing climate change research and provides a baseline for understanding and action. The Working Group I Summary for Policymakers released Sept. 27, 2013, states with greater certainty than ever that climate change is happening and that human activity is the principal cause. Among the highlights of the report:
- The conclusion that much of the warming over the past 50 years is due to human activities is now “extremely likely,” upgraded from “very likely” in the last report.
- Estimates of future sea level rise have been significantly increased due to a better understanding of the movement of ice sheets in a warming climate.
- The Arctic Ocean is now projected to be ice-free during the summer by mid-century under a high emissions scenario, instead of the end of century as in previous reports.
Each IPCC report has been progressively stronger in attributing climate change to human activities. The AR5 contains the strongest statement yet, saying it is “extremely likely” (a greater than 95 percent chance) that human activities are “the dominant cause of the observed warming” since the 1950s. The Third Assessment (2001) made a similar statement with approximately 66 percent certainty, while the Fourth Assessment Report (AR4) (2007) found that “most of the observed increase in global average temperatures since the mid-20th century is very likely (greater than 90 percent chance) due to the observed increase in anthropogenic greenhouse gas concentrations.”
The AR4 concluded that “warming of the climate system is unequivocal.” The AR5 goes further, concluding that many observed changes (warming of the atmosphere and ocean, sea level rise and melting ice) are “unprecedented over decades to millennia.”
New atmospheric temperature measurements in the AR5 show an estimated warming of 0.85 degrees Celsius (1.5 degrees Fahrenheit) since 1880 with the fastest rate of warming in the Arctic. The AR4 estimated the average warming across the globe over the past century (1906-2005) was 0.74 C (1.33 F).
Sea Level Rise
The AR5 report has significantly increased projected sea level rise over the next century, due to new research that improves understanding of ice sheet movement and melting. The new projections show an increase of 0.26-0.55 meters (10-22 inches) by 2100 under a low emissions scenario and 0.52-0.98 meters (20-39 inches) under the high emissions scenario. The AR4 did not include some of the effects of ice sheet movement due to warming, and therefore published much lower estimates in the range of 0.18-0.38 meters (7-15 inches) under a low emissions scenario and 0.26-0.59 meters (10-23 inches) under a high emissions scenario for sea level rise by 2100.
Sea and Land Ice
The AR5 projects it is likely (greater than 66 percent chance) that the Arctic Ocean will be ice-free during part of the summer before 2050 under a high emissions scenario. This represents a large shift from the AR4, which estimated that the Arctic Ocean would not be ice-free during the summer until late in the 21st century. The AR5 finds that Arctic sea ice surface extent has decreased by 3.5-4.1 percent per decade (9.4-13.6 percent during summer), which is higher than the AR4 estimate of 2.1-3.3 percent per decade (5-9.8 percent during summer). That amounts to between 0.45 and 0.51 million square kilometers (0.17 to 0.2 million square miles) per decade. The AR5 finds these changes unprecedented in at least the last 1450 years.
The AR5 also states that scientists have “high confidence” (80 percent chance) that glaciers have shrunk worldwide, and that the Greenland and Antarctic Ice Sheets have lost mass over the past two decades. The report notes with “very high confidence” (90 percent chance) that ice loss from Greenland has accelerated during the past two decades. Greenland is now losing about 215 gigatonnes (Gt) per year of ice, while the rest of the world’s glaciers lose about 226 Gt per year. For comparison, 200 Gt weighs the same as around 100 billion cars (about 1 billion cars exist on Earth today).
Surface Warming “Pause”
After a period of rapid warming during the 1990s, global mean surface temperatures have not warmed as rapidly over the past decade. The AR5 notes there are “differences between simulated and observed trends over periods as short as 10-15 years (e.g., 1998-2012)”. It concludes that the recent reduction in surface warming is probably due to a redistribution of heat in the ocean, volcanic eruptions, and the recent minimum in the 11-year solar cycle. Most importantly, the report specifically points out that these trends should not undermine our confidence in the “big picture” of our understanding of climate change: “trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends.”
In addition, there is new research proposing explanations for the recent trends that did not make the deadline to be included in the AR5. One paper suggests that some of this “lost” heat is actually in the deep ocean, while another notes that the warming “pause” is actually explained by the unusual number of La Niña (sea surface cooling events) in the Pacific Ocean. The second paper by Yu Kosaka and Shang-Ping Xie states that the “current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue.”
Cumulative Carbon Budgets
The AR5 relates different carbon “budgets” – an accumulated amount of carbon emissions over time — to the chances of average warming exceeding 2 degrees above 1861-1880 levels. Governments have set an international goal of limiting average warming to 2 C. For the world to have a 50 percent chance of staying below 2 C of warming by 2100, the AR5 identifies a greenhouse gas emissions budget of 840Gt of carbon. More than half of that (over 531GtC) has already been emitted. At current emission rates (around 10 GtC per year), we will use up our carbon budget in just 30 years.
Future Emission Scenarios
The report describes several alternative scenarios of 21st century greenhouse gas concentrations and global temperatures, each associated with different cumulative carbon budgets. Three scenarios represent potential pathways with less warming under various forms of mitigation policy. The fourth represents more of a business-as-usual case, with emissions in the 21st century three to four times larger than the emissions before the 20th century and the highest level of warming in any scenario.
Global surface temperature increases exceed 1.5 C and keep rising beyond 2100 in all scenarios except the lowest-emission scenario, in which actions are taken to nearly eliminate CO2 emissions in the second half of the 21st century. In the scenarios with higher rates of emissions, warming is likely to exceed 2 C by 2100, and could even exceed 4 C.