Changes in solar radiation dismissed as primary driver of climate change

Speaking at a seminar given by the US Global Change Research Program in Washington DC, the researchers were addressing the problem of how much of the observed climate change over the 20th century is attributable to changes in solar radiation, and how much can be attributed to changes in the concentration of greenhouse gases in the atmosphere resulting from human activities.

The researchers, Dr. Judith Lean, a research physicist at the Naval Research Laboratory, Washington DC and Dr. Jerry Mahlman, Director of the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory, Princeton, NJ, concluded that a larger role for the Sun in explaining climate warming over the 20th Century is inconsistent with measurements of solar output, and with evidence of variations in solar radiation during the pre-industrial era.

The researchers used model calculations to examine the effects of natural variability, carbon dioxide and other greenhouse gases, sulphate particles, and changing solar output on the climate of the 20th century. “In general,” the researchers said, “these calculations make it clear that it is scientifically very difficult to construct an explanation for the 20th Century warming that does not include a major role for the added greenhouse gases resulting from human activities.”

Based on the calculations, and the observational records of climate change for the 20th Century, the researchers drew the following conclusions:

• Global climate of the 20th Century has warmed by 0.7-0.8°C
• Natural climate variability cannot explain the magnitude of the observed warming over the 20th Century
• Variations in solar radiation are large enough to shape, but not dominate, the observed warming
• An extended warming period between 1910-1940 can be explained by natural variability plus added greenhouse gases. It can also be explained by added greenhouse gases plus increased solar radiation.
• Added greenhouse gases provide, by far, the most plausible hypothesis for explaining the warming of the 20th Century

The sun’s radiation, which helps determine fundamental climate state, is not constant. Changes in solar radiation must therefore be considered as a possible cause of climate change, among a number of other possible causes.

Space-based measurements reveal the existence of 11-year cycles in solar radiation. Unfortunately, direct observations of variations in solar radiation exist only for the last 20 years, which is a very short record in terms of climatological time scales.

Estimates of past solar activity have been calculated from tree-rings and ice-cores, known as proxy records. These also show the existence 11-year solar cycles as well as longer-term changes or cycles.

Comparisons of these proxy records with direct observations suggest that present levels of solar radiation are probably larger relative to periods of unusually low solar activity throughout history.

Solar activity increased steadily during the first half of the 20th Century, but since about 1950 the activity underlying the 11-year cycles has been essentially constant. There is little evidence since 1978 for an underlying upward trend in the Sun’s radiation, the researchers say.

Many types of climate parameters – such as temperature – -exhibit cycles that appear to correspond to solar activity cycles. This implies a consequential role for the Sun in climate change. Times of cooler climate in past millennia usually coincide with reduced levels of solar activity. During the Little Ice Age, which occurred from about 1450 to 1850, surface temperatures were from 0.6°C to 1°C colder than at present.

The speculated decrease of overall levels of solar radiation in 1650 from present levels had an effect on the climate of approximately 0.6 W/m². For comparison, the change in greenhouse gases since 1650 had an effect on the climate of about 2.5 W/m², 95% of which has occurred since 1850, according to the research.

Climate models with realistic sensitivities simulate surface temperature changes of a few tenths of a degree Celsius in response to changes in solar radiation over the past few hundred years. The researchers say the simulations can account for two hundred years of surface temperature fluctuations prior to the industrial epoch (from 1600-1800).

The solar-related warming in response to a solar forcing of 0.6 W/m² (from 1650 to the present) is calculated globally to be 0.45°C. Less than 0.25°C of this warming occurs from 1900-1990 when, for comparison, measured surface temperatures over this same period of time increased 0.6°C.

In suggesting that the Sun’s variability accounts for less than half the 0.6°C surface warming in the industrial period, the researchers say the climate change model simulations are in agreement with the pre-industrial empirical Sun-climate associations.

Therefore, say the researchers, assigning a major role for the Sun in explaining climate warming over the last 100 years, is inconsistent with direct measurements of solar output, and with proxy evidence of solar variability during the pre-industrial era.

During the past two decades (1976-1996) direct observations of solar radiation suggest negligible long-term solar forcing of climate. Over this observational period, solar radiation levels remained approximately constant, while observed surface temperatures nevertheless increased by 0.2°C over the same period of time.

Solar activity is presently at high levels relative to the historical record of the past 8,000 years. This suggests that future levels of solar radiation will probably be comparable to or less than present levels, and that future solar forcing will either be small, or negative, relative to the climate forcing due to greenhouse gases, while projected concentrations of greenhouse gases continue to increase.