Climate Change Impacts in the United States: The Third National Climate Assessment was published in May, 2014. It is the government’s official review and evaluation of climate change in the USA, focusing both on changes occurring now and on changes that can be expected throughout the remainder of this century. The full report is available for free over the Internet, and so is a “Highlights” document.

The report confirms that climate change is real and that it is human caused. The first graphic at right concerns global temperature from 1900 to 2005. The green band shows what temperature you would expect based on natural factors alone, the blue band shows what you would expect based on both natural and human factors, and the black line shows the actually observed temperatures. Somewhere around 1940, the blue and green bands start to diverge, with natural factors alone (the green band) simulating roughly constant temperatures, but combined human and natural factors (the blue band) simulating increased temperatures. The actually observed temperature tracks only one of these bands: the blue one. Climate change is human caused. (Chart from the full report, p. 8.)

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If we quickly curb our greenhouse gas (GHG) emissions, the average global temperature is predicted to rise 3-5°F by the end of the century, but 5-10°F if we don’t. In some regions, that doesn’t sound so bad. But if you are in a region that already has temperatures over 95 or 100°, that may not sound so good. The average global temperature has already risen by 1.3-1.9°F since 1895. The second graphic shows locations with temperatures above 100°F in 2011. The black circles denote weather stations that reported at least 1 such day. The colors show the number of days that temperatures were above 100°F. (Chart from the highlights, p. 24.)

(Click on map for larger view.)

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Many regions of the country have actually experienced increased precipitation. However, higher temperatures increase the rate of evaporation, causing the soil to dry out faster. As soil dries out, the heat of the sun goes into heating the soil rather than evaporating its moisture, resulting in even hotter and dryer conditions. In addition, even in areas where the total amount of precipitation is not projected to decline, more of it is projected to come during heavy downpours, leaving less to occur at other times. The result is a pattern of heavier precipitation, followed by longer dry spells in between. Large areas across the South, the Mississippi Valley, and the Great Plains already have significantly drier soil than 25 years ago. (See map at right, from full report p. 72.)

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Large portions of the country depend on melting snowpack for their primary water source, especially in the West. But increasing temperatures are likely to reduce the snowpack and cause it to melt earlier, leading to water shortages later in the year. In the graphic at right, the top map shows the projected decrease by mid-century in the amount of water locked up in the snowpack on April 1, assuming continued high GHG emissions. The middle map shows the projected decrease in runoff from April through June. The bottom map shows the projected decrease in soil moisture on June 1. (Graphic from full report, p. 71) As you wonder why the decrease in soil moisture is so much less than the other two decreases, remember that these are arid and semi-arid areas, and the soil is already sandy and dry. For a satellite photo of the 2014 snowpack compared to the normal snowpack, click this link: http://earthobservatory.nasa.gov/IOTD/view.php?id=82910.

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While one part of the country is drying out, other parts are experiencing increased flooding. River flood magnitudes have increased in the Northern Great Plains, much of the Mississippi and Ohio Valleys, and in the Northeast. These trends are expected to continue. The frequency and intensity of winter storms have increased, and their track has shifted northward. Trends in other severe storms, such as tornadoes, hail, and damaging thunderstorms, are uncertain. (Full report, p. 76)

Carbon dioxide is absorbed by the ocean from the atmosphere. Thus, as atmospheric levels of CO2 have increased, so have marine levels. In water, CO2 is converted to carbonic acid, leading to a gradual acidification of the ocean. Many marine creatures have shells made of calcium carbonate; acidic water prevents the shells from forming. Up until now, the oceans have been slightly basic, allowing the shells to form. But as it absorbs CO2, the ocean is becoming more acidic. In some regions shells have already become damaged or malformed, and as acidification continues, the effect will occur everywhere. There are large marquee animals such as crabs, shrimp, and lobsters that will be affected. But the most important effects will involve less notable creatures, such as krill. Krill are small animals that resemble tiny shrimp. A large fraction of marine animals depend on them as a food source, and if the number of krill declines, it will adversely impact life throughout the oceans. (Full report, p. 561)

Climate change will disrupt energy security through increased demand during summer peak load periods, severe weather events that damage significant portions of the grid and disrupt energy production. It is also predicted to cause problems with cooling water availability and quality, which will interfere with power generation. (Full report, p. 114)

Sea level rise, storm surge, and flooding will represent an increasing threat to transportation infrastructure in low-lying regions of the country. The effects will range from road closures due to flooding, to buckling due to high temperatures, to damage from extreme weather events. (Full report, p. 131)

Some agricultural regions will experience temporary increases in agricultural yields due to increased CO2 concentrations in the atmosphere and longer growing seasons. Eventually, however, most regions will experience reduced yields due to disruptions from heat stress, drought, and storms. Typically, agriculture is highly adaptive, but it is unknown if it will be able to adapt to these projected changes. (Full report, p. 151)

Climate change will have a negative impact on air quality through increased humidity, increased air pollution, increased allergens, and smoke from wildfires. In 2010, wildfires in Russia killed an estimated 56,000 people and caused $15 billion in damages. Fires in the American west have increased in frequency and severity. The photo at right shows smoke blanketing the Northeast from wildfires in Quebec in 2002. In the photo, the pure white is clouds. The yellowish grey is smoke. (Full report, p. 223)

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The report considers even more sectors, and it details many more ways climate change will impact the United States. This post is already too long, however, so I will stop here.

Sources:

Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp. doi:10.7930/J0Z31WJ2.

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Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Highlights of Climate Change Impacts in the United States: The Third National Climate Assessment. U.S. Global Change Research Program, 148 pp.

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Wikipedia. 2014. 2010 Russian wildfires. Viewed 6/12/2014 at http://en.wikipedia.org/wiki/2010_Russian_wildfires.