Surface Temperature Reconstructions for the Last 2,000 Years (2006) / Chapter Skim
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10 Climate Forcings and Climate Models
10 Climate Forcings and Climate Models
Pages 98-109
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From page 98... ...
· Climate model simulations indicate that solar and volcanic forcings together could have produced periods of relative warmth and cold during the preindustrial portion of the last 1,000 years. However, anthropogenic green house gas increases are needed to simulate late 20th century warmth.
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From page 99... ...
Climate models are often used to simulate the response of climate to variations in external forcing, including those indicated by proxy evidence. A variety of models have been used to estimate the surface temperature variations implied by the available proxy data for the last 2,000 years.
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From page 100... ...
, and right-hand axes show the radiative forcing relative to 1750 as calculated using the formulas given by Myhre et al.
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From page 101... ...
These orbital effects should have resulted in only a small trend in seasonal surface temperatures over the last 2,000 years, making Northern Hemisphere summers slightly cooler and Northern Hemisphere winters slightly warmer today than they were 2,000 years ago. Because of the seasonal nature of these effects, the incoming global average, annual mean solar radiation has remained essentially unchanged.
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From page 102... ...
2005a) indicate that solar irradiance has varied by about 0.1 percent over the last 2,000 years, which is equivalent to variations in radiative forcing of 0.10.2 W·m2 between periods of low and high solar activity (Figure 10-2)
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From page 103... ...
Farther back, polar ice cores in Greenland and Antarctica recorded the acidity and sulfate in annual ice layers from the settling of volcanic sulfate aerosols (Crowley 2000, Hegerl 2006)
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From page 104... ...
Peru 50 Vesuvius Krakatau Pinatubo 0 Early Medieval "MWP" 1 2 3 4 5 Cool Period Little Ice Age Cooling Phases -50 0 500 1000 1500 2000 Year AD FIGURE 10-3 Ice core estimates of global stratospheric sulfate loading from volcanoes (A.D.
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From page 105... ...
In addition, they can be used to estimate the sensitivity of the surface temperature and other climatic variables to the estimated climate forcings over this period and the uncertainties inherent in those estimates. Using model simulations with several temperature reconstructions and instrumental data over the past seven centuries and accounting for uncertainties in the reconstructions and forcings, Hegerl et al.
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From page 106... ...
For example, in some energy balance models and intermediate complexity models, volcanoes are represented as a negative deviation of the solar irradiance. Differences between the various model simulations can also be related to differences in the sensitivity of the models (Goosse et al.
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From page 107... ...
stronger cooling than the models that prescribe the volcanic forcing in terms of a reduction in the Sun's emission. ANTHROPOGENIC FORCING AND RECENT CLIMATE CHANGE Based on current estimates, variations in natural climate forcings over the last 2,000 years were much less than the increase in current greenhouse forcing due to human activities.
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From page 108... ...
compared to observational estimates (red line) with contributions of natural and anthropogenic forcings for 25-year periods shown as color bars.
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From page 109... ...
. Results using a Monte Carlo approach, climate model simulations, and observations of atmospheric and oceanic warming suggest a total radiative forcing from preindustrial times to the present of 1.42.4 W·m2, which is consistent with the observed warming (Knutti et al.
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