Today, the USGCRP released its Third National Climate Assessment, which is available for download or web browsing (see also coverage by the Capital Weather Gang). Given the release of the IPCC Fifth Assessment Report just a few months ago, the NCA doesn't contain anything all that surprising, but does provide yet another perspective on our warming world.
One of the key areas of improved scientific understanding in the past several years concerns the role of human and natural influences in driving global climate change. As summarized in Chapter 2: Our Current Climate:
"Natural drivers of climate cannot explain the recent observed warming. Over the last five decades, natural factors (solar forcing and volcanoes) alone would actually have led to slight cooling. 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."Multiple lines of evidence support this conclusion, including climate model simulations with and without human factors shown below.
|Observed global average changes (black line), model simulations using only changes in natural factors (solar and volcanic) in green, and model simulations with the addition of human-induced emissions (blue). Climate changes since 1950 cannot be explained by natural factors or variability, and can only be explained by human factors. (Figure source: adapted from Huber and Knutti). Figure and caption source: Walsh et al. (2014), The Third National Climate Assessment.|
- An increase in the average temperature of 1.3–1.9ºF since 1895 (most of this increase has occurred since 1970), with the last decade the nation's warmest on record.
- An increase in the frost-free growing seasons since the 1980s, with the largest increase over the western United States.
- An increase in heavy downpour frequency, especially in the last three to five decades with the largest increases in the midwest and northeast.
- More frequent and intense heat waves and less frequent and intense cold waves.
The report also notes that the intensity and duration of North Atlantic hurricanes, and the frequency of Category 4 or 5 hurricanes have increased since the early 1980s, but that the relative contribution of human and natural causes to these trends is uncertain. Trends in other severe storms (tornadoes, hail, damaging thunderstorm winds) are also uncertain.
One conclusion that I think may be misleading is "winter storms have increased in frequency and intensity since the 1950s, and their tracks have shifted northward over the United States." This statement is based on trends in extratropical cyclones, low-pressure systems that develop in the mid and high latitudes, and not trends in low temperature precipitation events that produce snow, sleet, or freezing rain, which is what I think of when I hear the term winter storm. Although I suspect the authors were trying to avoid the use of a jargony term like extratropical cyclone, this was an instance where they probably should have been more specific. To date, trends in the frequency of heavy snowstorms and seasonal snowfall show considerable regional variability and are difficult to generalize for the nation (this is discussed in the relevant sections of the report).
Perhaps we'll have a look at the projections for future climate change in a forthcoming post.