Solar Variability, Ozone, and Climate
Climate change may be caused by both natural variations and by human activities. It is therefore important to understand the relative influence of the various factors in order to estimate future climate changes and to decide how best to mitigate the negative impacts. People have thought for a long time that a major natural source of climate change are variations in solar output. For instance, these may have been responsible for episodes such as the Little Ice Age during the 15th to 18th centuries. As surface temperatures have risen rapidly during recent decades, as compared to the preceding decades, increasing attention has been paid to the sun's potential impact on Earth's climate, but the size of this impact remains very controversial. A first step towards understanding the long-term climate influence of solar variability is to unravel the effects of the well-observed 10-12 year solar cycle (which includes the well known sunspot cycle).
Previous studies have concluded that changes in solar output over a solar cycle seem to be too small to have much direct impact at Earth's surface. Solar cycle variability is greatest at ultraviolet wavelengths, which are largely absorbed by the stratospheric ozone layer. The direct effects of solar variability are therefore felt predominantly in the stratosphere or higher. However, since the stratosphere is coupled to the troposphere (the lower atmosphere), these changes could also indirectly affect the surface.
To understand the underlying physical mechanisms by which solar variability affects climate, as well as to assess the relative strengths of solar variability versus greenhouse gases, requires computer models of Earth's climate system. However, most climate models have concentrated on the lower atmosphere, and have not included the coupling between the stratosphere and the troposphere. We have now included both realistic solar irradiance and ozone changes in a version of the NASA Goddard Institute for Space Studies climate model which includes a representation of the complete stratosphere.
We find that changes in upper stratospheric ozone and winds affect the flow of energy at altitudes just below these changes, which then affect the next lower levels, and so on. The changes gradually work their way downwards, eventually altering the flow of energy in the lower atmosphere. The coupling between the stratosphere and lower atmosphere may therefore play a crucial role in the interaction between solar variability and climate. The total energy change over a solar cycle is quite small, which has led many to argue that solar variability has little impact on climate. Through this coupling, however, solar variability affects the lower atmosphere by changing the distribution of the large amount of energy which is already present. The impact on global average temperature seems indeed to be small; however, changing the flow of energy produces large regional impacts. To get a sense of the importance of the flow of energy on local climate, remember that Miami is warmer than New York primarily because it is closer to the equator and so receives more sunlight. Yet, Spain and southern Italy, lying at the same latitude as New York, have a warmer climate because of atmospheric and oceanic heat transport.
The solar induced changes in the lower atmosphere affect surface features such as temperature and pressure. The model's response agrees with observations, including the long record of geopotential height variations (a function of temperature throughout the lower atmosphere), implying that these observed 10-12 year oscillations are likely driven, at least partially, by solar variability.
It is intriguing to extrapolate these results to longer term solar irradiance changes, which are roughly two to three times larger than solar cycle variations. The pattern of modeled surface temperature changes induced by solar variability is well correlated with observed global warming over the first half of the 20th century, but not with the more rapid warming seen over the past three decades. The latter more closely resembles modeled warming induced by increasing greenhouse gas emissions. This suggests that although solar variability does impact surface climate indirectly, it was probably not responsible for most of the rapid global warming seen over the past three decades.
Shindell, D.T., D. Rind, N. Balachandran, J. Lean, and P. Lonergan 1999. Solar cycle variability, ozone and climate. Science 284, 305-308.