Why Has Global Temperature Rise Stalled?
Where has all the heat gone?
Eleven of the twelve warmest years on record have occurred since 2000. The average global temperature during this decade has also been the warmest on record, dating back about 130 years. But it has also been a period in which the mercurial rise of global average temperatures—prominent since about 1965 and before 1940—has flat-lined. The absence of a sustained temperature rise disreputes claims about global warming, decry many people skeptical of human-caused climate change. In a recent Op-Ed in the Wall Street Journal (WSJ), for example, a group of engineers and scientists stated there is no need to panic about global warming because “perhaps the most inconvenient fact is the lack of global warming for well over 10 years.”
Looking back at the historical record, numerous 10-year spans in which temperature trends declined are embedded within the longer-term increasing trend (Figure 1). While the rise in global average temperatures—the common index used to measure climate change—has indeed stalled in the last decade, it is likely not because man’s influence on the climate doesn’t exist. A better explanation, one with evidence set forth in a recent peer-review article, is that the incremental increases in energy caused by the concomitant rise in greenhouse gases (GHGs) is being stuffed at a faster rate into the deep oceans.
The article, “Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods,” published in the journal Nature Climate Change in September 2011, provides climate model evidence for why warming has sputtered in recent years.
According to Meehl and his coauthors, observations show that there is currently an energy imbalance in the climate system. About 1 watt of energy more per square meter (W/m2) is entering the atmosphere than leaving. This imbalance is translated through the climate system and would amount to about 0.75 degrees C (or 1.35 degrees F) of warming, on average, across the globe if the imbalance is maintained long enough. Measurements during the past half-century show that the global average temperature has indeed warmed by about this much (Figure 1).
Climate scientists know the energy is going somewhere other than the atmosphere because the imbalance continues and temperatures have not warmed in the last 10 years. The energy could be warming the land and/or melting ice or snow, but these sinks traditionally have played minor roles in the flow of energy. The best candidate is the ocean, which can suck up a lot of energy in the form of heat. The deep ocean in particular is a good sink and it can accumulate heat in numerous ways, including deceased sinking of Arctic surface waters during the winter and increased mixing of tropical water that allows warm surface water to sink.
It's a plausible, physically based explanation with one flaw: there are scant measurements of the deep oceans to test this hypothesis. The only viable tools are global climate models, which are often criticized because they incompletely represent reality. While this critique holds sway for some analyses, like forecasting monsoon precipitation in the Southwest, it has less merit when models produce results that mimic observations and processes and produce plausible outcomes, like they do in this case.
Meehl and his coauthors analyzed eight 10-year periods, from five different global climate models, in which globally averaged temperatures displayed decreasing trends. The state-of-the-art global climate models were driven by new GHG emission scenarios produced for the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) set for publication in 2014. It is considered a moderate scenario (called RCP 4.5). The evolution of GHG in this moderate scenario peaks at approximately 650 parts per million (carbon dioxide equivalent) around midcentury and falls afterwards (current carbon dioxide levels are at 392 parts per million). This represents more than a doubling of the GHG concentrations in the atmosphere since before 1850, and will likely increase the energy (which in turn controls temperature) within the climate system by 4.5 W/m2.
Results from Meehl and others suggest that during the decades when warming stalled, the average surface ocean waters cooled by about 60 percent relative to all other decades, while the ocean depths below 985 feet warmed.
Efforts are underway to boost observing capabilities in the deep oceans that will provide empirical evidence to corroborate and calibrate models. But in the meantime, the models still provide a plausible answer to where the heat has gone in the last decade; the study was even referenced in the rebuttal to the WSJ Op-Ed by leading climate scientists.