WHERE THE “MISSING HEAT” HAS GONE
The years since the turn of the 21st century have seen a temporary lull, or pause, in the rise of global atmospheric temperatures. Yet the causes of rising temperatures — especially the burning of fossil fuels that increases the atmosphere’s CO2 burden — are still with us. So where has the excess heat been going for the last decade or so, if not into the atmosphere? This question has generated a flurry of inquiry by climate scientists. The results of their recent research, added to an already vast store of data about global warming, point to the ocean’s depths as the repository of the missing heat.
A graph of the Global Land-Ocean Temperature Index clearly demonstrates the warming trend that has characterized the climate since the beginning of the 20th century — the ten warmest years of the 132-year record having occurred since 1998.
Following the 5-year running mean on the graph, we see that steep rises are offset at intervals by pauses, some brief and others more extended. In particular, we see a pause beginning around the turn of the 21st century and continuing into the present. Given the overall upward trajectory seen on the graph, we might reasonably expect that this current pause will end as they all have, and the rise in temperature will resume apace. In fact, climate scientists have been predicting this for the longer term. A statement issued in March of 2011 by Britain’s Meteorological Office reads,
“… even if greenhouse gas emissions are substantially reduced, the long lifespan of CO2 in the atmosphere means that we cannot avoid further climate change due to CO2 already in the atmosphere. […] Despite the uncertainties, all models show that the Earth will warm in the next century, with a consistent geographical pattern.”
This statement clearly affirms what is currently understood about future global warming.
A lull in the overall rise in atmospheric temperature may be newsworthy, and it has drawn a number of news outlets to seize upon it as if it were a new trend. The message in a recent Fox video broadcast is that climate change has stalled. That would certainly be good news for our planet, if it were true. But this message relies on a selective reading of the data, “global temperatures have remained flat over the last ten years” being the only reference to climate history in the broadcast. Such an interpretation singles out 10 years showing a short-term pause, while it ignores over 130 years of data showing the long-term rise in global temperatures.
Selecting only a portion of the data that conflicts with the data as a whole is cherry-picking, a tactic commonly used to misinform the public. As is typical of cherry-picking, the picture is incomplete and leads to a false conclusion. The whole of the data gives the complete picture: that atmospheric temperatures have been rising, despite pauses, for well over a century.
Over the decade or so of the current lull in temperature rise, climate scientists have continued to monitor Earth’s climate systems in an effort to discern how these pauses in a long trend of atmospheric warming occur. What they have come to understand is that the ocean’s depths have been absorbing the preponderance of the missing heat; in fact, the temperature rise in the deep ocean – below 700 meters – has even been accelerating since the turn of the 21st century. This acceleration in the rise of deep ocean temperatures contrasts with a deceleration in the rise of the ocean’s surface temperature, and the corresponding pause in the rise of atmospheric temperatures.
Source: Balmaseda et al., (2013)
During this pause, Earth’s overall climate is simply undergoing a redistribution of heat between the two systems, atmosphere and ocean. The ocean, by absorbing over 93% of the excess heat, strives to “catch up” to the previously faster-warming atmosphere. Warmed by the atmosphere, surface water is drawn down to the depths where it mixes with the colder water, gradually warming the depths. This dynamic overturning of waters also brings colder water up to the surface, where the temperature rise has seen a deceleration. Thus, the current pause in the rise of atmospheric temperatures does not mean that global warming has stalled, but rather that its locus is the depths of the ocean.
Retrospectively tested computer models show that for the rest of the 21st century, as atmospheric temperatures continue their rise, there will be other temporary pauses, or periods when temperatures level off, like the one we are experiencing now. Commenting on a study of such computer models conducted by the National Center for Atmospheric Research, climate researcher Kevin Trenberth speaks to this future: “This study suggests the missing energy has indeed been buried in the ocean. The heat has not disappeared, and so it cannot be ignored. It must have consequences.”
Reinforced by mounting scientific evidence, Trenberth’s warning tells us this current pause must not be construed as a reason to relax vigilance. We must continue to advocate for policies that discourage fossil fuel consumption and encourage the development of other sources of energy. Pauses in the inexorable rise in global temperatures are just that: only pauses – and they are part of the overall picture of continuing global climate change.