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The Role of Oceanic Heat Storage in Explaining Recent Observed Climate Changes Using GFDL Models
3rd Quarter GFDL Milestone - FY02
Purpose:
Previous observational work has documented a substantial increase in oceanic heat content from the middle 1950s to the middle 1990s. The purpose of this project is to use global ocean-atmosphere models to assess whether the observed increase in oceanic heat content is consistent with natural variability of the climate system, or whether it is indicative of anthropogenic climate change
Efforts:
Output from ensembles of numerical experiments with coupled ocean-atmosphere models were extensively analyzed and compared to observations. As a first step, it was demonstrated that the observed increase in ocean heat content does not occur in a long integration of the model with constant levels of greenhouse gases. This suggests that the observed increase is inconsistent with internal variability of the coupled system. It was then shown that an increase comparable to that observed does occur when the models are forced with estimates of the observed increases in greenhouse gases, sulfate aerosols, volcanic activity, and solar irradiance changes. These results suggest a role for human activity in generating the observed increase in ocean heat content.
A paper was published in Science describing these results.
Customers:
This work is of special interest and relevance to the climate change detection community as well as the general scientific community.
Significance:
The observed increase in oceanic heat content represents a large amount of net energy input into the ocean, and is a powerful indicator of climate change. Ocean heat content changes are a particularly useful diagnostic for detecting climate change, since they serve as a time and space integral of net fluxes of energy into the global ocean.
Success:
The agreement in long-term heat content trends between the model and observations (Figure 1) demonstrates the utility of such models for interpreting observed changes. Such retrospective studies are essential for evaluating the credibility of climate models and for reducing uncertainties in climate change projections.
Next Steps:
Current efforts are underway at GFDL to build the next-generation coupled Earth System models. These models will be used to conduct additional numerical experiments to improve our understanding of the mechanisms governing ocean heat content changes.
Figure 1. This figure shows the time series of observed and simulated global ocean heat content computed from the surface to 3000 m depth (in units of 1022 Joules). The black curve represents the observations, while the red curve is from simulations using the GFDL coupled ocean-atmosphere model. Specifically, the model curve represents the difference between a three-member ensemble of climate change experiments and an extended control integration in which greenhouse gases are kept fixed. In the climate change experiments, estimates are included of the radiative effects of past changes in greenhouse gases, solar irradiance, volcanic aerosols, and the direct effects of sulfate aerosols. There is a notable agreement in long-term trends of heat content between the observations and model, although the decadal scale fluctuations in the observed time series are not reproduced in the model.