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Clarifying some important issues about climate change

Scientific American
Date: July 15, 2007
Original URL

1) The relative magnitudes of solar versus anthropogenic forcing of the climate
There is now much better understanding of the relative roles of these forcing terms. For a start there are now decades-long, high-quality satellite solar measurements: These establish that there have been no significant changes in solar irradiance since 1979. In the longer term, reconstructions suggest a solar forcing since preindustrial times less than 10 percent that of the total human-induced forcing (Figure 2). In addition, the pattern of observed temperature changes-warming throughout the troposphere (the lowest 10 kilometers or so of the atmosphere), but cooling above that in the stratosphere-is inconsistent with solar-dominated forcing, but consistent with greenhouse gas (and other human-induced) forcing. There have been some suggestions that cosmic rays could affect clouds. However, these have been based on correlations using limited records. These have generally not stood up when tested with additional data, and furthermore, their physical mechanisms remain only speculative.

2) Surface and tropospheric warming records
The third assessment report (2001) noted an apparent mismatch between the instrumental surface temperature record (which showed significant warming over recent decades, consistent with a human impact) and the balloon and satellite atmospheric record (which showed little of the expected warming). This discrepancy is now largely resolved-with consistent warming now found between surface and atmosphere-following several new studies of the satellite- and balloon-derived atmospheric temperature record. These have corrected some significant biases and errors in the record, such as that caused by the "decay" of satellite orbits with time.

3) The role of water vapor in climate change
Although water vapor is the strongest greenhouse gas, direct human emissions are small, and its importance for climate change has long been known to arise from the way it responds to temperature changes. If forcing by another factor (such as CO2) produces a warmer atmosphere, this can hold more water vapor, further amplifying the warming. This feedback is critical in determining the overall magnitude of expected climate change. The latest Intergovernmental Panel on Climate Change (IPCC)[FIRST REF] report reaffirms these essential aspects. In particular there are new observations showing recent water vapor increases along the lines expected from observed warming trends as well as new evidence suggesting that models are getting their water vapor responses to warming about right.

4) The heating that is now clearly identifiable in the ocean
Warming has been observed not only averaged over land stations but also over the ocean. This underscores that the urban "heat island" effect, although real, is only local, because it cannot be present in ocean data. Indeed, it is found to be negligible in hemispheric or global averages. Improved and expanded ocean data also demonstrateR that a large amount of heat is being taken up by the ocean. This demonstratesR[OR, TO VARY: indicates] that the planet's energy budget has been pushed out of balance in our warming world, just as expected due to increases in greenhouse gases.

5) The new assessment of future sea level rise.

Estimates of future sea level rise from thermal expansion [see main text] are now taken from climate models that include a much more realistic simulation of ocean circulation compared with the simpler models used in the previous report (2001). Notably, however, central estimates for total climate-related sea level rise remain within 10 percent of previous estimates (when adjusted for the same time period). The modeled range, however, is now smaller (at both upper and lower ends) because of new constraints on the contributions from melting glaciers and ice caps and a more careful consideration of correlations between uncertainties in different terms. Importantly, the report gives no cause for any reduction in concern about future sea level rise. Recent observations show evidence for faster than expected breakup of ice around the edges of the Greenland and Antarctic ice sheets indicating that some of the processes causing ice sheets to discharge into the ocean in a warmer world are not included in current models. Thus, whereas the uncertainties in what can be modeled have been reduced, this is offset by the emergence of an additional factor for which no quantitative estimates exist in the literature.
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