After this six-part examination of Climategate, what do I, as an atmospheric scientist, believe?
There is nothing ominous about today’s temperatures. Temperatures were as warm, or warmer, during the Medieval Warm Period (MWP) and probably comparable in the Roman Warming. Al Gore’s ‘Hockey Stick’ which purports to show current temperatures are extraordinary and there is no MWP has been completely debunked.
That the earth’s “temperature” is actually lower than the CRU and NASA indices indicate but by an unknown amount. A reasonable guess (and it is just a guess) that that roughly a third of the warming in the NASA and CRU data since 1995 is due to instrumental changes and, perhaps, improper computer coding. The natural warming from 1915 to about 1945 may actually be greater than shown in the CRU and NASA data. That "natural" warming is comparable in magnitude to the allegedly human-caused “global warming” from 1979 to 1998.
Temperatures peaked in 1998. They have leveled off or fallen since then. This is confirmed by the lack of increase in heat content in the upper 2000 feet of the oceans which is more important than atmospheric temperatures as a metric of the planet’s temperature.
The IPCC models completely failed to forecast the current cooling and have no demonstrable skill at forecasting the future, especially on the regional scale.
CO2 is a greenhouse gas and, if everything else is equal, the atmosphere will warm as concentrations increase.
If everything is equal, the warming will be less than the IPCC projects because increasing concentrations of CO2 seem to act via a logarithmic relationship rather than direct (1 to 1) relationship. My judgment is that the IPCC is overstating the effect of CO2 by a factor of 3 or 4. Nevertheless, CO2-related warming could become worrisome if it continues enough decades into the future.
However, we do not understand the relationship of clouds, small particles (called aerosols by atmospheric scientists), and, especially, the sun to climate change. The IPCC models treat clouds and some of these other factors purely as contributors to global warming and this is likely not the case. So, we don’t know for sure what the net warming due to CO2 might be in the real atmosphere.
There is no question that humans affect the climate. This has been scientifically documented since the 1970’s with the Metromex Project in St. Louis. For example, if a forest is stripped out in favor of heavy urbanization, temperatures will become more variable, generally warmer, and downwind, more precipitation may occur. This is clear on the local and regional scale, but its significance on the planet-wide scale is unclear.
The sun has been showing signs of going into a “Maunder Minimum” or similar period of very low sunspot activity. In the past, these periods of very low solar activity have been associated with cooling climates, including the Little Ice Age. A number of scientific agencies around the world are warning of major cooling. Traditionally, humanity has done better in warm, rather than cold, climates. Note, however, that correlation does not equal causation and we don’t know for sure what caused these major coolings. I am agnostic as to the threat of major cooling.
There is no credibile evidence of an increase in storminess due to “global warming.” It is unlikely that the recent (but not unprecedented) low levels of summer ice in the Arctic is due to “global warming.” The more likely cause of the melting is the darkening of the ice cover due to increased soot pollution from Asia. There is no evidence that “global warming” is causing the glacier at Mt. Kilimanjaro to retreat.
Related note: A number of people have equated my scientific beliefs regarding global warming as somehow being in favor of the “status quo” with regard to energy policy. That is not the case.