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Report Shines Light on Dramatic Climate Changes, Reasons for Them


The National Climate Assessment summarizes the impacts of climate change on the U.S., now and in the future. To some extent, global climate change is addressed, as the U.S is not isolated from global impacts. A team of more than 300 experts guided by a 60-member Federal Advisory Committee produced the recent report, which was extensively reviewed by the public and experts, including federal agencies and a panel of the National Academy of Sciences.

The report is set up so that you can easily view sections individually. For example, there is a section on response strategies to climate change.

Climate change in the U.S. is different and more dramatic than in other countries around the world. Here are just a few highlights from the report:

Temperatures at Earth’s surface, in the troposphere (the active weather layer extending up to about 5 to 10 miles above the ground), and in the oceans have all increased over recent decades. The largest increases in temperature are occurring closer to the poles, especially in the Arctic. This warming has triggered many other changes to the Earth’s climate. Snow and ice cover have decreased in most areas. Atmospheric water vapor is increasing in the lower atmosphere because a warmer atmosphere can hold more water. Sea level is increasing because water expands as it warms and because melting ice on land adds water to the oceans.

Dramatic Drop in Sea Ice

Sea ice in the Arctic has decreased dramatically since the satellite record began in 1978. Minimum Arctic sea ice extent (which occurs in early to mid-September) has decreased by more than 40 percent. This decline is unprecedented in the historical record, and the reduction of ice volume and thickness is even greater. Ice thickness decreased by more than 50 percent from 1958-1976 to 2003-2008. The percentage of the March ice cover made up of thicker ice (ice that has survived a summer melt season) decreased from 75 percent in the mid-1980s to 45 percent in 2011.

U.S. average temperature has increased by 1.3°F to 1.9°F 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. 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.

While surface air temperature is the most widely cited measure of climate change, other aspects of climate that are affected by temperature are often more directly relevant to both human society and the natural environment. Examples include shorter duration of ice on lakes and rivers, reduced glacier extent, earlier melting of snowpack, reduced lake levels due to increased evaporation, lengthening of the growing season, changes in plant hardiness zones, increased humidity, rising ocean temperatures, rising sea level, and changes in some types of extreme weather.

The majority of the warming at the global scale over the past 50 years can only be explained by the effects of human influences, especially the emissions from burning fossil fuels (coal, oil, and natural gas) and from deforestation. Carbon dioxide has been building up in the atmosphere since the beginning of the industrial era in the mid-1700s, primarily due to burning coal, oil, and gas, and secondarily due to clearing of forests. Atmospheric levels have increased by about 40 percent relative to pre-industrial levels.

Methane Levels Have Increased by 250 Percent

Methane levels in the atmosphere have increased due to human activities including agriculture (with livestock producing methane in their digestive tracts and rice farming producing it via bacteria that live in the flooded fields); mining coal, extraction and transport of natural gas, and other fossil fuel-related activities; and waste disposal including sewage and decomposing garbage in landfills. Since pre-industrial times, methane levels have increased by 250 percent.

Other heat-trapping gases produced by human activities include nitrous oxide, halocarbons, and ozone. Nitrous oxide levels are increasing, primarily because of fertilizer use and fossil fuel burning. The concentration of nitrous oxide has increased by about 20 percent relative to pre-industrial times.

The conclusion that human influences are the primary driver of recent climate change is based on multiple lines of independent evidence. The first line of evidence is the fundamental understanding of how certain gases trap heat, how the climate system responds to increases in these gases, and how other human and natural factors influence climate. The second line of evidence is from reconstructions of past climates using evidence such as tree rings, ice cores, and corals. These show that global surface temperatures over the last several decades are clearly unusual, with periods warmer than any time in at least the last 1,300 years and perhaps much longer.

The third line of evidence comes from using climate models to simulate the climate of the past century, separating the human and natural factors that influence climate. When the human factors are removed, these models show that solar and volcanic activity would have tended to slightly cool the earth, and other natural variations are too small to explain the amount of warming. Only when the human influences are included do the models reproduce the warming observed over the past 50 years.

The report is very comprehensive and detailed and worth a read as you put together the reasons for your sustainability strategies moving forward.