Can human activity have led to global cooling?

Human activity may be behind the decrease in global temperatures from 1945 to 1975, a period during which a significant increase in small particles were released into the atmosphere. Over the last 20-30 years, anthropogenic emissions (mainly greenhouse gases) have in all likelihood resulted in global warming (IPCC, 2001). However, recent studies also show that human activity, both in the previous century and before, can actually have led to a drop in global temperatures. Govindaswamy et al. (2001) demonstrate that changes in vegetation and thus the albedo of the earth's surface (that is, the ability of the earth to reflect solar radiation) can have resulted in the observed cooling from the year 1000 to 1900. We recently published an article (Myhre et al., 2001) where we show that small particles in the atmosphere, known as aerosols, may have had a cooling effect on global temperatures during the mid-1900s. In this study, we looked at the development of anthropogenic radiative forcing (see textbox) from the most well known climate mechanisms in the period 1850-1995: carbon dioxide and other well mixed greenhouse gases in the atmosphere; changes in the amount of ozone both in the troposphere and the stratosphere; and the role of a number of aerosols. In addition to these anthropogenic sources, we also studied two natural radiative forcings. Greenhouse gases First we present the results from the various calculations of the anthropogenic radiative forcings. We calculated the radiative forcing of the greenhouse gases by using observed changes in the concentrations of these gases and combining this with what we know about their physical characteristics. The results are shown in Figure 1. Note that the radiative forcing from carbon dioxide increased throughout the entire period, except for a short decrease after 1940. The concentration of ozone varies both in time and space in the