Heat Storage within the Earth System

MARCH 2003 AMERICAN METEOROLOGICAL SOCIETY | T his commentary describes how an examination of the global heat budget allows a straightforward explanation for understanding one of the consequences of human changes in the composition of the earth’s atmosphere. Data and analysis provided in Levitus et al. (2000, 2001) on increases in heat stored within the world’s oceans provide a unique opportunity to explore this perspective. The use of a global heat budget to assess this consequence of human perturbations of the earth system was also introduced in Pielke (2001b). This note expresses the Levitus et al. data in terms of long-term, globally averaged values of heat flux (W m−2), and relates the fluxes to the radiative forcing of the earth’s climate system. These fluxes provide a constraint on estimates of radiative forcing such as provided by the Intergovernmental Panel on Climate Change (IPCC). Such an assessment of the global heat budget was provided in Peixoto and Oort (1992), based on the study of Ellis et al. (1978). In this contribution, it is concluded that the IPCC would more effectively depict changes over time in the climate system by using a heat balance perspective in order to diagnose the earth’s radiative imbalance. The application of such a perspective will require new priorities in global climate monitoring that are outlined in the conclusions. Radiative forcing is defined by the IPCC as “a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth– atmosphere system, and is an index of the importance of the factor as a potential climate change mechanism. It is expressed in W m−2.” The IPCC presents estimates of the change in radiative forcing of the climate system between 1750 and 2000 (Houghton et al. 2001; presented in Fig. 3 of the Statement for Policymakers and reproduced in this paper as Fig. 1). A continuous rate of 1.43 W m−2 of radiative forcing, for example, would correspond to a transfer of 2.33 × 1023 J of energy per decade into the climate system. The Levitus et al. data provide an opportunity to assess the portion of this radiative forcing that is actually warming the earth system. To do this, the Levitus et al. ocean data is reported in W m−2. Expressing their data in this manner provides a constraint on the net radiative forcing that results from the terms listed in Fig. 1. HEAT STORAGE WITHIN THE EARTH SYSTEM