Wednesday, May 7, 2014

National Climate Assessment: The Future

The National Climate Assessment released yesterday (see the previous post) summarizes some of the key trends that are expected to emerge or accelerate during the 21st century.

It's no surprise that warming is expected to continue in the coming decades, with an increase in the frequency and intensity of heat waves and a decrease in the frequency and intensity of cold waves.  This is consistent with recent climate trends, our understanding of the climate system, studies of past climate, and climate-model projections.

The magnitude of the warming is dependent on future greenhouse gas emissions.  The figure below shows the change in average annual temperature from 1970–1999 to 2071–2099 projected by climate models based on scenarios with rapid greenhouse gas emission reductions (left) and continued emissions increases (right).

These reports are sometimes called by some as "alarmist," but the summary statement in Chapter 2: Our Changing Climate is actually subdued.
"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."
For a summary statement, it strikes me that they should have been more specific about the magnitude of the warming (with assessment of likelihood) and the fact that warming and related changes to the climate system will likely continue well beyond 2100 unless we reduce greenhouse gas emissions dramatically and rapidly.  For example, here's what the IPCC said in their 2013 Fifth Assessment Report.  RCP2.6 is the rapid greenhouse gas reduction scenario, with 4.5, 6.0, and 8.5 representing low, moderate, and high emission scenarios.
"Global surface temperature change for the end of the 21st century is likely to exceed
1.5°C relative to 1850 to 1900 for all RCP scenarios except RCP2.6. It is likely to exceed 2°C for RCP6.0 and RCP8.5, and more likely than not to exceed 2°C for RCP4.5. Warming will continue beyond 2100 under all RCP scenarios except RCP2.6. Warming will continue to exhibit interannual-to-decadal variability and will not be regionally uniform."
Water is the agent that delivers climate-change impacts, to projections for changes in precipitation, including the average climate and the intensity and frequency of storms, are also quite important.  As summarized in the assessment, precipitation projections for the nation, based on an average of many climate models, call for drier future conditions across most of the southern continental US and wetter future conditions across most of the northern continental US.  Hatched areas indicate that the trends are significant and consistent amongst the models.
Those with an astute eye can probably infer that Utah lies in the transition zone between the wetter and drier climate regimes in an area that is not hatched, indicating a lack of consistency amongst the models.  Although it is often said that the Southwest is expected to dry with climate change, trends for Utah, especially northern Utah, are a bit less clear.  Due to a "reved up" water cycle, heavy precipitation events are expected to increase in frequency and intensity.

The big wildcard in this climate assessment game is sea level rise.  This can easily be ignored when you live in a high-altitude state like Utah.  Although some say that there will be winners or losers with climate change, but everyone loses with sea level rise.  Sea level rise stems from the warming of the oceans, which results in thermal expansion, and the loss of land-based ice.  As discussed in the sea level rise section of the assessment, thermal expansion alone is projected to produce a rise of about 11 inches in low emissions scenarios, with high emissions scenarios yielding larger rises.

The challenge, however, is estimating the rate of ice loss from the Greenland and Antarctic ice sheets, which house the vast majority of the land-based ice.  Thanks to the GRACE satellite, we now have remarkable estimates of ice mass loss from Greenland and Antartica.  At present, both are showing declines that if sustained would add about 18 inches to sea level rise by 2100 (this is in addition to increases from thermal expansion and other land-based ice).
The challenge, however, is that the record is short and efforts are needed to determine if these trends are due to long-term changes or short-term variability and how they will change in the future.  Ultimately, the authors of the National Climate Assessment conclude that a foot of sea level rise is on the low end of possibilities, with four feet plausible on the high end, although they note that decision makers might want to use a wider range of 8 inches to 6.6 feet.
Of course, sea level rise will not stop in 2100.  Although not highlighted in the report, one of the more important passages in my view is the following.
"Sea level rise will not stop in 2100 because the oceans take a very long time to respond to warmer conditions at the Earth’s surface. Ocean waters will therefore continue to warm and sea level will continue to rise for many centuries at rates equal to or higher than that of the current century. In fact, recent research has suggested that even present day carbon dioxide levels are sufficient to cause Greenland to melt completely over the next several thousand years.
Once the train leaves the station, it will be impossible to stop.

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