Simulating the 1951-2050 Climate with an Atmosphere-Ocean Model
We aim to learn something about the role of the ocean in climate change by using successively more complex representations of the ocean in simulations of the climate of the past 50 years and the next 50 years.
The three ocean representations we employed were 1) observed sea surface temperatures and sea ice; 2) an ocean with empirically-specified horizontal heat transports, called the Q-flux ocean; and 3) a three-dimensional dynamical ocean model called HYCOM. Our simulations were all driven using climate forcings previously described by Hansen et al. (2002).
All three models yield reasonable agreement with observed surface air temperatures in the past 50 years. The modeled heat storage between 1951 and 1998 ranges matches well with the observational data of Levitus et al. (2000). Still, it is difficult to define the observed change of ocean heat content precisely because of poor spatial sampling in the early 1950s and the large apparent variability throughout the 1950s.
For the period 2000-2050 we applied two climate forcing scenarios: a "business-as-usual" (BAU) scenario, which yielded a forcing of ~2.9 Watts per square meter (W/m2), and an "alternative" scenario with slowing growth rates of greenhouse gases, which yielded an increase of 1.1 W/m2.
In the HYCOM ocean simulation, the stratosphere cools almost 1°C in the BAU scenario for the years 2000-2050, but only a few tenths of a degree in the alternative scenario. Conversely, the troposphere and surface warm by only 0.3-0.4°C in the next 50 years in the alternative scenario but by more than 1°C in the BAU scenario. In both cases, the temperature changes obtained using the HYCOM ocean model are less than those obtained with the Q-flux ocean, because more heat is stored in the HYCOM ocean than in the Q-flux ocean.
Increased sequestering of heat by the ocean not only reduces surface warming in coming decades, it also increases Earth's energy imbalance. Even with the moderate climate forcing of the alternative scenario and a low climate sensitivity of 2.4°C for a doubling of atmospheric carbon dioxide, the net flux of energy into the planet increases to ~1.3 W/m2 in 2050. The fact that more of the greenhouse heating is sequestered and less appears as near-term warming is a consequence of the deep mixing and long response time of the more realistic ocean representation of the HYCOM model.
There are some consistent features in the geographical patterns of simulated heat storage with the HYCOM ocean model that imply possible regional effects of climate change in the coming half-century. In both the BAU and alternative scenarios, the heat storage along the West Coast of the Americas is increased. Similarly the model consistently yields a region of decreased heat storage in the Pacific Ocean between about 30°N and 60°N. These patterns have been observed during recent decades and are often suggested to be cyclical. However, our simulations suggest that a tendency to have these patterns may be a consequence of the forcings. Thus, these ocean temperatures may be a harbinger of climate patterns that will tend to exist in coming decades, rather than being dynamical fluctuations.
The simulations for 1950-2000 using the model with observed sea surface temperatures and sea ice provide evidence that Earth was out of radiation balance in 1951 by an estimated ~0.2 W/m2. The current rate of change of ocean heat content provides a measure of the "residual" global climate forcing, i.e., the existing forcing of the global climate system, which is estimated to be between 0.5 and 1.0 W/m2. This residual forcing implies additional global warming is "in the pipeline" and will occur without any further change of atmospheric composition. This warming is also reflected in the simulations of both the Q-flux and HYCOM ocean models.
It is desirable to have more complete ocean measurements, with better geographic and depth sampling in the coming decades, since they will provide an invaluable indicator of the climate system and will help refine our knowledge of the planetary energy imbalance — which is important for determining future global climate change. It's also important to check the changes in the ocean's heat distribution in order to predict the geographical distribution of climate change. A more complete dataset of ocean heat storage also can be used to check the ocean models' abilities to simulate the distribution of heat anomalies and provide much needed guidance for mixing parameterization in ocean modeling.
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