Nov 7, 2008
Climate response to solar forcing less than reported
While recent studies have found that changes in the amount of sunlight reaching Earth have caused 65% of surface warming over the last 100 years, a new study puts that figure at just 10%. The researchers found that, if anything, in the past 25 years solar forcing has caused a very slight overall cooling, not 20-30% of the warming.
"The influence of the Sun, in particular, versus anthropogenic influences on global change is extremely controversial and these recent articles have provoked this controversy further, evoking major consternation and confusion amongst scientist and the policy-making community," Judith Lean of the Naval Research Laboratory, US, told environmentalresearchweb. "Our own recent work analyzes the entire historical surface temperature record, using the IPCC anthropogenic and solar forcings simultaneously with ENSO [El Niño-Southern Oscillation] and volcanic influences. In this way we attempted to account simultaneously for the major forcings known to impact climate, whereas the prior studies that concluded a dominant role for solar variability neglected one or more of the other known influences."
Lean, who worked with David Rind of NASA, reckons their results clearly show that natural changes cannot account for the significant long-term warming in the historical global surface temperature anomalies, and further strengthen the likelihood that the recent warming is anthropogenic in origin.
The pair says their findings suggest deficiencies in general circulation models used to simulate and forecast climate change. "We showed that climate appears to respond to solar variability more rapidly – within months versus years – and with larger amplitude, by a factor of four or five, than the models simulate," explained Lean. "Contrary to model-based expectations, the 11-year solar cycle is reliably detected in the historical surface temperature record. Surface temperature responds rather rapidly, in phase with and lagging only by a month or so, the solar irradiance changes."
The researchers believe that dynamical processes are involved rather than, or as well as, thermodynamic processes. "Existing circulation patterns – Hadley, Ferrel, and Walker cells – are likely engaged, along with atmosphere – ocean interactions (ENSO)," said Lean. "Furthermore, there are likely both direct (surface heating) and indirect (stratospheric influence) components. Current models are not able to represent these processes very well; many lack, for example, adequate stratospheres and ENSO representations." Lean hopes that future validation of models with the pair’s empirical results may lead to improvements, which would then enable better estimates of responses to greenhouse gases.
Lean and Rind also carried out the first comparison from observations of the geographical distributions of responses to the individual forcings. This produced the first estimate of the geographical pattern of ENSO and volcanic aerosols over the entire globe. "IPCC  did not report geographical patterns of climate responses to individual natural and anthropogenic influences because of model uncertainties at smaller than continental scales and over timescales less than 50 years," said Lean. "Our derived regional response patterns presage the next IPCC (AR5), which plans to focus more on regional responses compared with the 2007 focus on global attribution."
The researchers found that anthropogenic warming appears to have a much smaller signal at northern high latitudes than the IPCC reports, and than general circulation models simulate, at least in the annual averages. "However, our analysis is confined to latitudes less than about 65°N, where more reliable surface temperature observations exist, and captures only that variability at high latitudes which linearly tracks the four forcings (at appropriate lags)," said Lean.
Next Lean and Rind plan to determine the seasonal variation in regional responses to different climate influences; to extend the analysis to the atmosphere above the surface rather than just looking at surface temperatures; to compare empirical and modeled regional response patterns in detail; and to work on forecasting surface temperatures in the next decade by looking at the response to solar and greenhouse gas changes in the context of plausible scenarios for ENSO and volcanic activity, which are difficult to forecast.
"We have completed a large suite of model simulations using the GISS Middle Atmosphere GCM of climate range in the last 50 years (see Rind et al., JGR, in press)," said Lean. "Preliminary analyses indicate significant differences between the modeled and observed temperature responses to both solar and anthropogenic forcings, which we plan to elucidate and quantify, annually and seasonally, at the surface and in the atmosphere."
The researchers reported their work in Geophysical Research Letters.