Third National Climate Assessment: Evidence, Impacts Of Warming Abound

Providing the U.S. government and public with its most thorough and comprehensive “status report on climate change science and impacts” to date, the National Climate Assessment and Development Advisory Committee (NCADAC) on May 6 released the final version of the Third National Climate Assessment report (NCA-3).

“From the top of the atmosphere to the depths of the oceans ... and in the observed and measured changes in location and behavior of species and the functioning of ecosystems,” evidence that our climate is warming -- and the resulting impacts across U.S. society and geography -- abound today, according to NCA-3. “Taken together, this evidence tells an unambiguous story: The planet is warming, and over the last half century, this warming has been driven primarily by human activity." The main culprit, report authors add, is our burning of fossil fuels.

Moreover, meticulous observations of some key climate indicators, such as sea-level rise and arctic sea ice melt, are changing faster than the best climate models envisioned. Realization of the worst future climate scenarios can still avoided, but much greater reductions in carbon and greenhouse gas emissions will have to be achieved at a much faster rate than is taking place at present, NCA-3 authors conclude.

U.S. and global climate trends

The average temperature in the U.S. has increased 1.3 to 1.9 degrees Fahrenheit since record-keeping began in 1895, NCA-3 highlights, with most of the increase occurring since about 1970. The warming trend is expected to continue, with the decade just past being the nation's warmest on record to date. And as NCA-3 report authors note: “Because human-induced warming is superimposed on a naturally varying climate, the temperature rise has not been, and will not be, uniform or smooth across the country or over time.”

In the “Highlights of Climate Trends” section of the report, the report authors state:

“Global climate is changing and this change is apparent across a wide range of observations. The global warming of the past 50 years is primarily due to human activities. Global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades depends primarily on the amount of heat-trapping gases emitted globally, and how sensitive the Earth's climate is to those emissions.”

A comprehensive, integrated assessment of climate change

In addition to the integrated analysis of climate change and impacts it provides, by comparing actual observations made across the U.S. with predictions from climate system models, the NCA-3 also serves as a “gauge” of greater scientific knowledge and improved understanding of the nature, mechanisms and potential impacts of our warming climate.

Looking to broaden, as well as deepen, our understanding of climate change and its socioeconomic and ecological impacts, NCA-3 incorporates advances in climate science “into larger social, ecological, and policy systems, and with this provide integrated analyses of impacts and vulnerability.” It also drills down to offer analyses of climate change and impacts across 10 U.S. regions, from Alaska and Hawaii to Maine and Florida, as well as in rural areas and along U.S. coasts.

The result of a four-year long production process that included public comment and inter-agency government review, the NCA, though based on scientific research, is intended to inform and guide public policy across every sphere of U.S. society.

Analyzing future climate scenarios and impacts

Based on actual observations and the best climate models scientists have to offer, the third NCA extends well beyond scientific research. Incorporating actual climate observations and calibrating them with the best climate models available, NCA authors created a wide range of climate scenarios. They then assessed the potential ecological and socioeconomic impacts of these scenarios across the geography of the U.S.

In the report's introduction, NCA authors provide a broad summary of the results and conclusions that are explained in greater detail in the 12 sections of the report. Encouragingly, they state that scientists' understanding of Earth's complex climate -- and humanity's role in shaping and influencing it -- has improved greatly in recent decades. On the other hand, observations of some key climate change indicators are changing at faster rates than those envisioned by climate models.

The NCA report authors write, “With each passing year, the accumulating evidence further expands our understanding and extends the record of observed trends in temperature, precipitation, sea level, ice mass, and many other variables recorded by a variety of measuring systems and analyzed by independent research groups from around the world.

"It is notable that as these data records have grown longer and climate models have become more comprehensive, earlier predictions have largely been confirmed. The only real surprises have been that some changes, such as sea level rise and Arctic sea ice decline, have outpaced earlier projections.

“What is new over the last decade is that we know with increasing certainty that climate change is happening now. While scientists continue to refine projections of the future, observations unequivocally show that climate is changing and that the warming of the past 50 years is primarily due to human-induced emissions of heat-trapping gases. These emissions come mainly from burning coal, oil, and gas, with additional contributions from forest clearing and some agricultural practices."

Producing the Third National Climate Assessment

A 60-person Federal Advisory Committee, the NCADAC, was created by the Department of Commerce in 2010 “to oversee the activities of the National Climate Assessment.” Supported by the National Oceanic and Atmospheric Administration (NOAA), NCAs are produced every four years as per the Global Change Research Act of 1990 -- a $2.6 billion federal research program on global change.

In producing the Third National Climate Assessment, the NCADAC enlisted more than 300 expert contributors. Following the January 2013 release of a draft version of the report, more than 4,000 public comments were received and reviewed by the authors and National Academies.

The resulting Government Review Draft, along with “derivative highlights,” was then submitted for formal inter-agency review. The NCADAC adopted the final version of the report and Highlights and submitted them to the federal government for publication and distribution on May 6.

In order to provide a thorough, comprehensive and integrated assessment of climate change and its impacts, NCA authors created scenarios based on a wide range of assumptions about population growth, economic development, technological evolution, and environmental policy and regulatory decisions, among other factors.

Quantitative and narrative descriptions of “plausible future conditions,” these scenarios served as inputs to the analysis of potential impacts and responses to climate change. Encompassing climate and sea level changes, land use and socioeconomic conditions, the scenarios weren't associated with probabilities, and hence they aren't predictions or forecasts. Rather, the NCADAC elaborates, they “provide a range of future conditions to bound uncertainty.”

NCA-3's 12 key messages

NCA-3 report authors highlight 12 “Key Messages” that correspond to the report's 12 chapters and spotlight key climate trends and their potential impacts across U.S. society and geography.

1. Observed Climate Change: Global climate is changing and this change is apparent across a wide range of observations. The global warming of the past 50 years is primarily due to human activities.


2. Future Climate Change: Global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades depends primarily on the amount of heat-trapping gases emitted globally, and how sensitive the Earth’s climate is to those emissions.


3. Recent U.S. Temperature Trends: U.S. average temperature has increased by 1.3 to 1.9 degrees Fahrenheit since record keeping began in 1895; most of this increase has occurred since about 1970. The most recent decade was the nation’s warmest on record. Temperatures in the United States are expected to continue to rise. Because human-induced warming is superimposed on a naturally varying climate, the temperature rise has not been, and will not be, uniform or smooth across the country or over time.


4. Lengthening Frost-free Season: The length of the frost-free season (and the corresponding growing season) has been increasing nationally since the 1980s, with the largest increases occurring in the western United States, affecting ecosystems and agriculture. Across the United States, the growing season is projected to continue to lengthen.


5. U.S. Precipitation Change: Average U.S. precipitation has increased since 1900, but some areas have had increases greater than the national average, and some areas have had decreases. More winter and spring precipitation is projected for the northern United States, and less for the Southwest, over this century.


6. Heavy Downpours Increasing: Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.


7. Extreme Weather: There have been changes in some types of extreme weather events over the last several decades. Heat waves have become more frequent and intense, especially in the West. Cold waves have become less frequent and intense across the nation. There have been regional trends in floods and droughts. Droughts in the Southwest and heat waves everywhere are projected to become more intense, and cold waves less intense everywhere.


8. Changes in Hurricanes: The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. The relative contributions of human and natural causes to these increases are still uncertain. Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate continues to warm.


9. Changes in Storms: Winter storms have increased in frequency and intensity since the 1950s, and their tracks have shifted northward over the United States. Other trends in severe storms, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds, are uncertain and are being studied intensively.


10. Sea Level Rise: Global sea level has risen by about 8 inches since reliable record keeping began in 1880. It is projected to rise another 1 to 4 feet by 2100.


11. Melting Ice: Rising temperatures are reducing ice volume and surface extent on land, lakes, and sea. This loss of ice is expected to continue. The Arctic Ocean is expected to become essentially ice free in summer before mid-century.


12. Ocean Acidification: The oceans are currently absorbing about a quarter of the carbon dioxide emitted to the atmosphere annually and are becoming more acidic as a result, leading to concerns about intensifying impacts on marine ecosystems.

Images courtesy of the NCADAC Third National Climate Assessment