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.
With the Intergovernmental Panel on Climate Change (IPCC) poised to release its Fifth Assessment of the science underpinning our understanding of climate change, it’s useful to take a step back and recap some of the “big picture” facts.
What is already clear from the science:
- Carbon dioxide and other greenhouse gases act to warm the planet.
- Carbon dioxide is accumulating in the atmosphere due to emissions from human activities.
- The Earth has been warming during the past century. The amount and speed of the warming is unusual compared to past records.
- Humans’ emissions of greenhouse gases are largely responsible for this warming.
- If emission rates continue, the warming in the 21st century will be much more significant than the warming in the previous century.
In its periodic assessments, the Intergovernmental Panel on Climate Change has expressed growing certainty that global warming is underway and that human activity is a principal cause. The panel’s language has become progressively stronger over time to reflect its growing certainty.
In 1990, the IPCC said that emissions from human activities were “substantially increasing” greenhouse gas concentrations in the atmosphere, which would lead to warming.
By 2013, the panel had concluded that "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."
The chart below traces how the IPCC’s conclusions have strengthened over time.
|Year||Assessment||Statement on Human Involvement in Climate Change|
“…emissions resulting from human activities are substantially increasing atmospheric concentrations of greenhouse gases…
These increases will enhance the greenhouse effect, resulting on average in an additional warming of the Earth’s surface”
“Most of these studies have detected a significant change and show that the observed warming trend is unlikely to be entirely natural in origin…
…the balance of evidence suggests that there is a discernible human influence on global climate.
…the average rate of warming [in projections for the 21st century] would probably be greater than any seen in the last 10,000 years, but the actual annual to decadal changes would include considerable natural variability.”
“There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.
…the projected rate of warming is much larger than the observed changes during the 20th century and is very likely to be without precedent during at least the last 10,000 years, based on paleoclimate data.”
“Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level.
Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
"Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased.
…It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century."
In 2009, the Obama Administration convened the Interagency Climate Change Adaptation Task Force, and the President signed Executive Order 13514, directing agencies to improve energy and water efficiency, better manage waste and pollution, and reduce greenhouse gas emissions. In addition, the Order requested that agencies identify vulnerabilities and put together a climate adaptation plan by June 2012. The plans were released in February 2013 and began implementation for FY 2013. These Adaptation Plans are often part of an agency’s broader Sustainability Plan and will be updated each year.
Highlighted Adaptation Plans
Other Federal Agency Adaptation Resources
Other federal agencies have published climate change adaptation plans as directed by Executive Order 13514. These agencies are either smaller or have provided fewer details in their adaptation plans; links to the plans are below:
Mayor Michael Bloomberg’s $20 billion plan to safeguard New York City against a future Hurricane Sandy and other climate risks is the most ambitious effort yet by any U.S. city to prepare for the expected impacts of climate change.
The mayor last week announced “A Stronger, More Resilient New York,” a comprehensive plan to protect communities and critical infrastructure, and proposed significant changes to New York’s building codes for new construction and major renovations that will help buildings withstand severe weather and flooding. Its 250 recommendations include building new infrastructure (like installing armor stone shoreline protection in Coney Island), changing how services are provided (like encouraging redundant internet infrastructure), and establishing standardized citywide communication protocols for use during disruptions.
Hurricane season officially starts June 1 and it looks like a busy one in the Atlantic. The National Oceanic and Atmospheric Administration (NOAA) expects a well above-average hurricane season with 13 to 20 named storms. Seven to 11 of them could develop into hurricanes and three to six of those could be major (defined as category 3 or higher). The average over about the past 30 years is 12 named storms, six hurricanes, and three major hurricanes per season.
Photo courtesy NOAA
Tornadoes and Climate Change
Definition of a TornadoTornadoes are formed by a combination of atmospheric instability and wind shear. Instability occurs when warm, moist air is wedged under drier, cooler air aloft. This warm air rises, causing the intense updrafts and downdrafts seen in strong thunderstorms — the incubators of tornadoes. Wind shear refers to changes in wind direction and speed at different elevations in the atmosphere. The combination of instability and wind shear forms the rotating column of air that we associate with a tornado. Tornadoes that form over water are known as waterspouts.
The link between tornadoes and climate change is currently unclear. One problem is the difficulty in identifying long-term trends in tornado records, which only date back to 1950 in the United States. Also, the population in many areas affected by tornadoes has grown, so it’s possible that tornadoes in the early part of the 20th century occurred without anyone seeing them. Improved technology, such as advanced radar, also helps us “see” tornadoes that may not have been detected decades ago.
Another problem lies with the physics associated with tornadoes. Researchers are working to better understand how the building blocks for tornadoes -- atmospheric instability and wind shear -- will respond to global warming. It is likely that a warmer, moister world would allow for more frequent instability. However, it is also likely that a warmer world would lessen chances for wind shear. Recent trends for these quantities in the Midwest during the spring are inconclusive. Climate change also could shift the timing of tornadoes or the regions that are most likely to be hit, with less of an impact on the total number of tornadoes.
Adding to the difficulty, tornadoes are too geographically small to be well simulated by climate models. Models can simulate some of the conditions that contribute to forming severe thunderstorms that often spawn tornadoes. Multiple studies (see here and here) find the conditions that produce the most severe thunderstorms are likely to occur more often in a warmer world, even if the total number of thunderstorms decreases (because of fewer weak storms). However, this work does not conclusively tell us whether tornadoes should follow the same trend as their parent thunderstorms.
Threats posed by tornadoes
The most significant threats from tornadoes are the dangers posed by strong winds and debris that is caught up in those winds. Although individual tornadoes may affect a relatively small area compared to large tropical storms, they can threaten people, homes, and communities.
NOAA estimates that, on average, about 1,200 tornadoes occur across the country annually, but several hundred more or fewer tornadoes can occur in any given year.
On average, tornadoes in the United States cause 70 deaths and 1,500 injuries per year. The death toll from tornadoes has dropped rapidly because forecasters have more tools to detect dangerous weather and quickly warn people to take shelter.
However, tornadoes still cause billions of dollars a year in property damage. The costliest year on record for tornado damage was 2011, when seven tornado and severe weather outbreaks each caused more than $1 billion in damages, and the total damage for the year was more than $28 billion.
How to Build Resilience
Communities can bolster their resilience and reduce the impacts from tornadoes by:
- Adopting more stringent building codes in tornado-prone areas
- Continuing to support new severe weather research and improvements to forecasts for severe weather
Heeding watches and warnings when they are issued, and ensuring that individuals can be reached by emergency alert systems (for example, through text message, television, and radio, or via tornado sirens)
To Learn More
Last updated July 2016
Extreme Weather Event Map: Click on any circle to learn about one of the billion-dollar weather events, or any state to learn about billion-dollar droughts. All events occurred between 2000 and May 2016.
Floods, Tornadoes, Thunderstorms, Hail, Tropical Storms, Wildfires, and Winter Storms are all shown as circles, with the costs indicated by the area of the circles (see image to the right). The location of the circles correspond to places where impacts were experienced (note: locations are approximate; many of the events actually impacted a large area, beyondThis map shows billion-dollar weather events in the United States since 2000, as identified by the National Oceanic and Atmospheric Administration’s National Climatic Data Center. The Top 10 costliest events are listed at the bottom of this page, along with a description of major U.S. droughts since 2000.
the boundaries of the circle). Droughts are not shown by circles, but by the shading in the states – states with darker colors have experienced more droughts since 2000, while states that are lightly shaded have experienced fewer droughts. No billion dollar events have occurred in Hawaii since 2000; some of the wildfire impacts (e.g., fire seasons in 2006, 2007, and 2008) included damages in Alaska, but the markers appear in the continental United States.
Many of these events, including heat waves and heavy rainfall, are likely to become more frequent and intense as a result of climate change. Climate change can also worsen the impacts of some of these events. For example, sea level rise can increase the impacts of coastal storms and warming can place more stress on water supplies during droughts. But it’s important to note that not ALL of these events will necessarily happen more frequently as a consequence of climate change. The links between climate change and tornadoes, ice storms, and hail are unclear, and represent current areas of research.
These events demonstrate ways our communities and infrastructure are vulnerable to extreme weather, and that the costs associated with impacts can be large.
More Resources on Extreme Weather and Climate Change
Fact Pages: Learn more about the links between climate change and:
Weathering the Next Storm - Extreme weather is costly. The events shown on the map above all cost billions of dollars, and several events had widespread and long-lasting implications.
A 2015 C2ES Report, Weathering the Next Storm: A Closer Look at Business Resilience, examines how companies are preparing for climate risks and what is keeping them from doing more. It also suggests strategies for companies and cities to collaborate to strengthen climate resilience. The report synthesizes public disclosures by S&P Global 100 companies, in-depth interviews and case studies, and workshops. It updates the groundbreaking 2013 report, Weathering the Storm, Building Business Resilience to Climate Change, which provided a baseline for how companies were assessing their climate vulnerabilities.
- Preparing for more summer heat waves (July 2016)
- Drought in California (June 2014)
- Extreme Weather and resilience: Coverage from a Senate hearing on resilience (Feb. 2014)
- The polar vortex (Jan. 2014)
- Some lessons from Hurricane Sandy (Nov. 2013)
- Coastal flood risks (April 2013)
|Event and Date||Cost||Fatalities||Description|
|$148 billion||1,833||The hurricane initially hit as a Category 1 near Miami, FL, then as a stronger Category 3 along the eastern LA-western MS coastlines, resulting in severe storm surge damage (maximum surge probably exceeded 30 feet) along the LA-MS-AL coasts, wind damage, and the failure of parts of the levee system in New Orleans. High winds and some flooding occurred in Ala., Fla., Ga., Ind., Ky., Miss., Ohio and Tenn.|
|$65.7 billion||159||Sandy caused extensive damage across several northeastern states (Conn., Del., Mass., Md., N.J., N.Y., R.I.) due to high wind and coastal storm surge, particularly in N.J. and N.Y. Damage from wind, rain and heavy snow also extended more broadly to other states (N.C., N.H., Ohio, Pa., Va., W.Va.), as Sandy merged with a developing Nor'easter. Sandy interrupted critical water and electrical services in major population centers and caused 159 deaths (72 direct, 87 indirect). Sandy also shut down the New York Stock Exchange for two consecutive business days, which last happened in 1888 due to a major winter storm.|
|$30.0-$30.3 billion||123||The 2012 drought was the most extensive in the U.S. since the 1930s. Moderate to extreme drought conditions affected more than half the country for a majority of 2012. Costly impacts included widespread harvest failure for corn, sorghum and soybean crops, among others. The associated summer heat wave also caused 123 direct deaths, but the excess mortality due to heat stress is still unknown.|
|$29.2 billion||112||Ike made landfall in Texas as a Category 2 hurricane. It was the largest Atlantic hurricane on record by size, causing a considerable storm surge in coastal TX and significant wind and flooding damage in Ark., Ill., Ind., Ky., La., Mich., Mo., Ohio, Pa., Tenn. and Texas. Severe gasoline shortages occurred in the Southeast due to damaged oil platforms, storage tanks, pipelines and refineries.|
|$19 billion||35||The Category 3 hurricane hit SW Florida, resulting in strong damaging winds and major flooding across southeastern Florida. Prior to landfall, Wilma as a Category 5 recorded the lowest pressure (882 mb) ever recorded in the Atlantic basin.|
|$19 billion||119||The Category 3 hurricane hit Texas-Louisiana border coastal region, creating significant storm surge and wind damage along the coast, and some inland flooding in the Fla. panhandle, Ala., Miss., La., Ark., and Texas. Prior to landfall, Rita reached the third lowest pressure (897 mb) ever recorded in the Atlantic basin.|
|$18.5 billion||35||The Category 4 hurricane made landfall in southwest Florida, resulting in major wind and some storm surge damage in FL, along with some damage in the states of S.C. and N.C..|
|$17.2 billion||57||The Category 3 hurricane made landfall on Gulf coast of Ala., with significant wind, storm surge, and flooding damage in coastal Ala. and Fla. panhandle, along with wind/flood damage in the states of Ga., Miss., La., S.C., N.C., Va., W.Va., Md., Tenn., Ky., Ohio, Del., N.J., Pa., and N.Y.|
|$12.0-$12.4 billion||95||In Texas and Oklahoma, a majority of range and pasture lands were classified in "very poor" condition for much of the 2011 growing season.|
|$11.1 billion||48||The Category 2 hurricane made landfall in east-central Fla., causing significant wind, storm surge, and flooding damage in FL, along with considerable flood damage in the states of Ga., N.C., N.Y. and S.C. due to 5-15 inches of rain.|
Table 2: Drought Events since 2000
|2015||$4.5 billion||0||Drought conditions continued to affect California throughout 2015, heavily impacting the agricultural sector. Drought conditions inproved in Texas and Oklahoma due to several major flood events.||Ariz., Calif., Idaho, Mont., Nev., Ore., Utah, Wash.|
|2014||$4 billion||0||Historic drought conditions affected the majority of California for all of 2014, making it the worst drought on record for the state. Surrounding states and parts of Texas, Oklahoma and Kansas also experienced continued severe drought conditions. This is a continuation of dought conditions that have persisted for several years.||Ariz., Calif., Kan., Nev., N.M, Okla., Ore., Texas.|
|2013||$11 billion||53||The 2013 drought slowly dissipated from the historic levels of the 2012 drought, as conditions improved across many Midwestern and Plains states. However, moderate to extreme drought did remain or expand into western states. In comparison to 2011 and 2012 drought conditions the US experienced only moderate crop losses across the central agriculture states.||Ariz., Calif., Colo., Idaho, Kan., Neb., Nev., N.M., Okla., Ore., S.D., Texas, Utah, Wyo.|
|2012||$30.0-$30.3 billion||123||The 2012 drought was the most extensive drought to affect the U.S. since the 1930s. Moderate to extreme drought conditions affected more than half the country for a majority of 2012. Costly drought impacts occurred across the central agriculture states resulting in widespread harvest failure for corn, sorghum and soybean crops, among others. The associated summer heatwave also caused 123 direct deaths, but an estimate of the excess mortality due to heat stress is still unknown.||Ariz., Ark., Calif., Colo., Ga., Idaho, Ill., Ind., Iowa, Kan., Minn., Mo., Mont., Neb., Nev., N.M., N.D., Okla., S.D., Texas, Utah, Wyo.|
|2011||$12.0-$12.4 billion||95||Drought and heat wave conditions created major impacts for affected areas. In Texas and Oklahoma, a majority of range and pastures were classified in "very poor" condition for much of the 2011 crop growing season.||Ariz., Kan., La., N.M., Okla., Texas|
|2009||$5.0-$5.4 billion||0||Drought conditions occurred during much of the year across parts of the Southwest, Great Plains, and southern Texas causing agricultural losses in numerous states. The largest agriculture losses occurred in Texas and California.||Ariz., Calif., Kan., N.M., Okla., Texas|
|2008||$2.0-$2.2 billion||0||Severe drought and heat caused agricultural losses in areas of the South and West. Record low lake levels also occurred in areas of the Southeast.||Calif., Ga., N.C., S.C., Tenn., Texas|
|2007||$5.0-$5.6 billion||15||Severe drought with periods of extreme heat over most of the Southeast and parts of the Great Plains, Ohio Valley, and Great Lakes area reduced crop yields, stream flows and lake levels.||Ala., Ark., Fla., Ga., Ill., Ind., Iowa, Kan., Ky., La., Mich., Minn., Miss., Neb., N.Y., N.C., N.D., Ohio, Okla., Pa., S.C., S.D., Tenn., Texas, Va., W.Va., Wis.|
|2006||$6.0-$6.9 billion||0||Severe drought affected crops in the Great Plains and across portions of the South and far West.||Ala., Ark., Calif., Colo., Fla., Ga., Iowa, Kan., La., Minn., Miss., Mo., Mont., Neb., N.M., N.D., Okla., S.D., Texas, Wyo.|
|2005||$1.0-$1.2 billion||0||Severe localized drought caused significant crop losses, especially for corn and soybeans.||Ark., Ill., Ind., Mo., Ohio, Wis.|
|2002||$10.0-$12.9 billion||0||Moderate to extreme drought was experienced over large portions of 30 states, including the West, Great Plains, and much of the eastern U.S.||Ala., Ariz., Calif., Colo., Conn., Del., Fla., Ga., Idaho, Iowa, Kan., La., Maine, Md., Mich., Miss., Mo., Mont., Neb., Nev., N.J., N.M., N.C., N.D., Ohio, Okla., Ore., Pa., R.I., S.C., S.D., Texas, Utah, Va., Wyo.|
|2000||$4.0-$5.4 billion||140||Severe drought and persistent heat over south-central and southeastern states caused significant losses to agriculture and related industries.||Ala., Ariz., Ark, Calif., Colo., Fla., Ga., Idaho, Iowa, Kan., La., Miss., Mont., Neb., Nev., N.M., N.C., Okla., Ore., S.C., Tenn., Texas, Utah, Wash., Wyo.|