tions are increased by the cultivation of rice, and nitrous oxide concentrations are increased by the use of nitrogen fertilizers in agricultural activity.
Natural concentrations of tropospheric aerosols remained relatively constant before the industrial period. Concentrations increased over the 20th century due to human activities (Ramaswamy et al. 2001). The radiative forcings of tropospheric aerosols have large ranges associated with uncertainties in aerosol sources, composition, and properties and their interactions with clouds (NRC 2005). Observations and models indicate that, in total, the direct effects of aerosols and the indirect effect associated with aerosol–cloud interactions likely lead to a reduction of solar radiative flux at the Earth’s surface, although separate aerosol species, such as black carbon, may have a positive radiative forcing (Ramaswamy et al. 2001). Tropospheric aerosols persist in the atmosphere for days to weeks and have significant regional variation; uncertainties in their forcing effects are large, especially at smaller spatial scales (NRC 2005).
Computer models can be used to simulate the behavior of the climate system, taking into account both temporal and geographic variability, to understand both the natural variability of the climate system and the response of the climate system to changes in climate forcings (NRC 2001). These simulations can also be used to interpret proxy-based climate reconstructions (Trenberth and Otto-Bliesner 2003). A hierarchy of models have been used to simulate the climate variability of the last 1,000 years. These models range from simpler energy balance models to much more detailed and computer-intensive models of the Earth system. Climate models have been used to test various aspects of surface temperature reconstructions. 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. (2006) give an estimate of climate sensitivity of 1.5–6.2°C.
Climate simulations of the last 1,000 years have been performed with a variety of models (Jones and Mann 2004b and references therein). These simulations generally indicate that surface temperatures in the Northern Hemisphere were cooler prior to the 20th century than during the reference period from 1961 to 1990, with the second half of the 20th century being warmer than any part of the preceding millennium (Figure 10-4). Simulated Northern Hemisphere surface temperatures decrease from A.D. 1000 to a broad minimum extending from 1500 to 1700, and they warm substantially after 1900. Coolings associated with volcanoes are evident in the simulated surface temperatures. The average response of a climate model to volcanic forcing exhibits an e-folding time of cooling consistent with that inferred from large-scale tree-ring- and multiproxy-based reconstructions of Northern Hemisphere summer surface temperature (Hegerl et al. 2003; Figure 10-5). The model simulations are consistent with published surface temperature reconstructions.