The Sun vs. the Volcano: Drivers of Regional Climate Change
"Global" warming is widely discussed, yet it is the changes at the regional and local level that people will actually feel. These can sometimes be quite different from global average changes. Variations in circulation patterns in the atmosphere and/or ocean that transport heat may warm some areas while cooling others, with only marginal changes to the global average temperature. To understand the many facets of climate change, it's important therefore to examine regional changes in addition to those at hemispheric or global scales.
Observations of Earth's climate are most complete since about 1850, but over this same period human activities have changed many aspects of the atmosphere that influence climate. This makes the past 150 years extraordinarily complex and difficult to understand. But prior to the industrial revolution, climate change was simpler. Variations over decades to centuries were driven only by volcanic eruptions and solar variability. Since a large amount of data on surface temperatures exists for the past few centuries, it can be very fruitful to study the period from about 1600 through 1850 as a complement to modern climate change.
In a recent study (Shindell et al. 2003), we examined the global and regional response to volcanic eruptions and solar variability using the NASA Goddard Institute for Space Studies climate model, and compared the results with reconstructions of surface temperatures from historical times. We focused on the change between the late 17th and 18th centuries. The late 17th century was one of the coldest parts of the period known as the "Little Ice Age", when hemispheric average temperatures dropped by a few tenths of a degree. Both reduced solar output and numerous large volcanic eruptions could have contributed to the hemisphere-wide cooling. Regional changes were much larger, however. Europe and eastern North America, for example, were unusually cold, while some other areas appear to have been relatively warm.
In the climate model, both the reduced output from the sun and the increased amount of volcanic aerosols (fine particles which reflect sunlight away from the Earth and which are injected into the stratosphere by volcanic eruptions) contribute a sizeable amount to the Northern Hemisphere cooling seen in temperature reconstructions. Their relative importance depends upon uncertain estimates of exactly how much each changed relative to a century later.
Regionally, however, solar forcing appears to have played the dominant role for the annual average over periods longer than a year or two (see figure at right). There are several reasons for this. Volcanoes do have a large impact on circulation of the atmosphere for a year or two, but the aerosols fall out of the atmosphere subsequently, resulting in little long-term effect. Furthermore, during the first and second years following eruptions, westerly air flow increases around the Northern Hemisphere during winter, when circulation responds most strongly to climate change, and brings relatively warm oceanic air over the continents. In the summer, however, eruptions lead to cooling via their reflection of sunlight, and this is strongest over the continents. The regional effects of eruptions thus tend to cancel one another, leading to a fairly uniform annual average response (see the figure).
In contrast, reduced solar output can last for decades and has similar impacts in all seasons. It leads to reduced wintertime westerly flow, resulting in colder continental temperatures. These are reinforced by cooler temperatures everywhere in the summer due to the reduced solar energy, but again the land responds more than the oceans. So during both seasons the land cools in response to solar forcing, leading to a large regional annual average response.
As shown in the figure, the annual average response to solar forcing resembles the observations much more closely than the response to volcanic forcing. The response is overall too strong, however, which may indicate either an overestimate of how much the sun dimmed during the late 1600s, an oversensitive model, or both. Nevertheless, it is clear that solar variability seems much more likely to have driven the large regional climate changes seen in historical data. This provides both a useful test of the ability of climate models to simulate patterns of regional changes and important evidence for the importance of solar variability in climate change.
Shindell, D.T., G.A. Schmidt, M.E. Mann, D. Rind, and A. Waple 2001. Solar forcing of regional climate change during the Maunder Minimum. Science 294, 2149-2152.
Shindell, D.T., G.A. Schmidt, R.L. Miller, and M.E. Mann 2003. Volcanic and solar forcing of climate change during the preindustrial era. J. Climate 16, 4094-4107.