PrintClimate is Already Changing
The impacts of increasing greenhouse gas concentrations are already being felt around the globe, though the degree of change varies with location. The Third National Climate Assessment (NCS), released in 2014 by the US Global Change Research Program (USGCRP), reports that over the last century increasing average temperatures, increasing weather variability, increasing warmer nights and winters, lengthening of the growing season, and an increase in the frequency and intensity of extreme weather events have already been observed. The severity of these impacts varies throughout the US and the world because of regional topography, proximity to the ocean, atmospheric circulation patterns, and many other factors.
Changing Temperature Patterns
The average temperature in the United States has increased in the last century, with each recent decade being warmer than the past, but this warming is not uniform across the United States (Figure 9.1.5). In general, western and northern regions have warmed more than the southeastern US. In the most recent decade, all regions have shown warming. What impact might this warming trend have on our food production and water supply? For example, we know from our study of water for food production that plants evaporate or transpire water and that the rate of evaporation is dependent on temperature. If temperatures go up, we know that plants will transpire more water. The southwestern US is already a water-scarce area, so increasing temperatures will exacerbate that condition.
We'll explore more connections between climate change and food production in the next section of this module. First, let's investigate changes in some other climate variables.
Changing Precipitation Patterns
In addition to changing temperatures, the recent decades have seen changes in precipitation patterns. Nationwide average precipitation has increased (Figure 9.1.6), but the patterns of change are not as clear as those for temperature. Notice in Figure 9.1.6 that the water-scarce Southwest experienced a decline in precipitation in recent decades. Additionally, some of the precipitation increase in the eastern US came in form of extremely heavy precipitation (Figure 9.1.7) and resulted in flooding (Figure 9.1.8). Both of these effects are anticipated results of increased concentrations of heat-trapping greenhouse gases in the lower atmosphere.
