Is Global Warming Spatially Complex?

The short answer is Yes.

As discussed in Heat storage within the Earth system, the appropriate climate metric to assess global warming is ocean heat content in Joules. As was shown in that 2003 paper, the radiative imbalance of the climate system can be effectively assessed by monitoring changes in Joules of the ocean heat content over time, as the other stores of heat in the climate system are small. For example, in that paper, between the mid-1950s and the mid-1990s, a global radiative imbalance of + 0.3 Watts per meter squared was diagnosed, with half of this heating (+0.15 Watts per meter squared) above 300 m and the remainder between 300 m and 3 km. Since that study, the analysis of Willis et al. 2004 provides more recent ocean heat storage changes. As we diagnosed from their data (see Pielke and Christy), the radiative imbalance for the period 1993- mid 2003 was about 0.62 Watts per meter squared.

However, these estimates are based on a global ocean average heat storage change. The actual spatial trends in ocean heat content are actually quite complex (i.e., see Figure 4 in Willis et al. 2004). They found that most of the heating was in the southern hemisphere mid-latitude ocean down to a depth of 750 m. The sea surface temperature (SST) anomalies mirror this spatial complexity at the surface of the oceans (see http://www.osdpd.noaa.gov/PSB/EPS/SST/climo.html for a current map of the anomalies). On September 24, 2005 large areas of cool SST anomalies are evident in the southern hemisphere oceans, while large areas of warm SST anomalies are seen in the northern hemisphere Atlantic Ocean.

An advantage of using Joules as the climate metric of global heat changes is that an adequately sampled snapshot at any moment of time is all that is needed to monitor the heating within the climate system. Unlike surface air temperature by itself (that has been the main climate metric used to assess global warming), in which there is a lag between a radiative imbalance and an equilibrium temperature; e.g. see Equation 1-1 in NRC 2005: Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties ), there is no lag between a radiative imbalance and the amount of Joules in the climate system.

We can, therefore, apply an assessment of the current anomalies in ocean heat content to determine where the global warming signal is most pronounced. Data provided by the European Centre for Medium Range Forecasting of near-current ocean heat content anomalies (presented as ocean temperature anomalies) can be used to illustrate the current spatial complexity of the ocean heating. The ECMWF presents data for the following slices through the oceans:

1. Equatorial depth temperature anomaly

2. Latitude –depth temperature anomaly at 165E

3. Latitude –depth temperature anomaly at 140W

4. Latitude –depth temperature anomaly at 109.7W

5. Latitude –depth temperature anomaly at 30W

The near-surface temperature anomaly (5 m depth) is also available from the ECMWF.

There is an issue as to whether all of the important spatial scales of the heat anomalies are sampled in these analyses. Nonetheless, there are several important conclusions from even a cursory examination of these slices even if we still need improved spatial monitoring.

A significant portion of the warming is at depth. The portion of this heat that is a depth below the thermocline is not readily available to heat the atmosphere above or to contribute to enhanced evaporation of water vapor from the ocean surface. This heat is “sequestered” for an unknown period of time.

The anomalies have significant horizontal, as well as vertical variations. Such horizontal structure could be a result of the heterogeneous character of a number of the climate forcings, as we discussed in the weblog entry for July 28, 2005 (What is the Importance to Climate of Heterogeneous Spatial Trends in Tropospheric Temperatures?) and/or related to the complexity of ocean dynamical and thermodynamic processes. A number of these anomalies are cooler than the long-term average.

Thus, the answer to the question posed in this weblog is that global warming has significant spatial variations. Global warming is not a more-or-less uniform warming spread across the oceans. Such a spatially complex warming pattern further supports the claim that a multiple set of climate forcings, in addition to the more homogeneous radiative forcing of the well-mixed greenhouse gases, is altering our climate. The reconstruction of the observed temporal evolution of the spatial pattern over the last several decades by the global climate models remains an unrealized goal.

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