In the typical story of climate change, greenhouse gases – predominantly from emissions associated with burning fossil fuels – are released into the atmosphere and warm the earth. As we continue to emit more greenhouse gases, Earth’s average surface temperature should increase. But over the past decade, global surface temperatures have plateaued even as greenhouse gases have continued to increase . How should we interpret this apparent pause, or ‘hiatus’, in global warming? Could it mean greenhouse gases aren’t bad after all? Is climate change over? While scientists agree that the answer to both of those questions is ‘No’, the precise explanation is up for debate.
Figure 1: Although levels of greenhouse gases have been steadily rising, global surface temperatures have showed little increase over the past decade. This plateau in temperatures has been referred to as a hiatus. Adapted from NASA Earth Observatory .
The Many Effects of an Energy Imbalance
The use of surface temperature as the sole metric for the effect of greenhouse gases emissions on climate is slightly misleading. A changing climate also leads to rising sea levels and melting ice. Although surface temperature has plateaued, sea levels have continued to rise and ice sheets have continued to melt at a faster pace. For instance, the West Antarctic ice sheet has started to fall apart and has likely reached a tipping point beyond which further melting is inevitable [2, 3]. Greenhouse gases impact all of these different metrics of climate change by modifying Earth’s energy budget.
At its heart, climate change is really an issue of energy imbalance: more energy is coming into the Earth than is going out. Greenhouse gases are largely responsible for this imbalance because they trap sunlight and tilt the scales toward greater incoming energy than outgoing energy. Part of this extra energy in Earth’s system melts ice and warms the air, which is what we see when we look at changes in surface temperature. However, the vast majority of this excess energy is absorbed by the oceans as heat. In fact, the expansion of ocean water as it warms is a contributor to rising sea levels (as, of course, is melting ice) [2, 4].
One possible explanation for the global warming hiatus relies on the large energy absorption capacity of the oceans. Water has a high heat capacity, meaning that it can absorb a lot of energy with small changes in temperature. When you consider the vast amount of water in the oceans, it adds up to a lot of energy storage potential. If the same amount of energy it takes to raise the temperature of the whole ocean 0.1°C (0.18°F) were instead absorbed by the atmosphere, it would raise air temperatures by about 100°C (180°F) ! The ocean, however, doesn’t increase in temperature uniformly: because the ocean circulates slowly, it takes a while before heat is mixed into the deep waters. It has been suggested that during the hiatus, changes in ocean circulation have led to the faster mixing of heat into deep ocean waters, allowing the ocean to take up more of the energy that would otherwise be channeled towards increasing surface temperatures. Ocean circulation follows an established geographical pattern. There are only a few locations in the ocean where warm surface waters sink into the deep ocean and where cold deep waters are ventilated to the surface – such as in high latitude ocean regions and along the equator in the Pacific Ocean – and they do so at a slow, steady rate. Increasing the rate of water exchange or expanding these areas would allow for more warm surface water to mix with cold, deep water, reducing the apparent surface temperature change .
If this is the case, it raises some interesting questions: What caused the circulation to change? Is it a permanent shift? If circulation patterns return to normal, would some of that stored heat be returned to the atmosphere, and would we expect to see an accelerated increase in surface temperatures?
Even as rising greenhouse gas concentrations increase the energy absorbed by the Earth system, a number of other factors impact the energy budget in the opposite direction. For instance, clouds reflect incoming sunlight and cause the earth to absorb less energy. Altogether, these factors are called climate forcings. These different forcings are playing tug of war, with some factors causing Earth to emit more energy, cooling the climate, and others trying to force the earth to absorb more energy, warming the climate .
As the earth warms up, the amount of thermal (long-wave) radiation it emits will increase. Eventually the earth will warm enough that even with greenhouse gases trapping some of the thermal radiation, the amount released to space will again be enough to balance the short wave radiation absorbed by the earth. At that point, the earth will be in equilibrium again and its average temperature will stop rising.
Figure 2: Climate forcings work to either warm or cool the earth by trapping or emitting more energy. Aerosols and clouds work to cool the earth, while greenhouse gases (such as ozone, carbon dioxide, and methane) work to warm the earth. Earth’s energy balance is the sum of these positive and negative forcings.
While we know that the warming side is winning the tug of war, we don’t know the exact score—that is, the size of Earth’s energy imbalance. Most of the uncertainty stems from the unknown cooling strength of small particles in the atmosphere called aerosols, a broad term that includes any solid or liquid particle, such as haze and dust. (Aerosols from spray cans that contributed to the ozone hole are just one type – there are many other natural and man-made sources of aerosols that don’t impact ozone.) These aerosols reflect sunlight and promote cloud formation, increasing the amount of energy the earth emits.
Recent increases in coal consumption in Asia (where regulations on power plant aerosol emissions are not as stringent as in Europe and North America) have changed aerosol levels in the atmosphere, and it is possible that the aerosols are pulling harder toward the cooling side than expected. If that is the case, then the energy imbalance is less than expected and the hiatus is the result of a smaller increase in energy of the Earth system. Unfortunately, the warming side of the tug of war will keep on getting stronger as greenhouse gases build up in the atmosphere. Burning coal also emits greenhouse gases, and in the long term, its warming effects will win out over the cooling effects of aerosol emission. Even though aerosols might currently be able to keep the cooling side in the fight, the warming side will eventually overpower them and increase surface temperatures .
Hiding in Plain Sight
Another possible explanation behind the hiatus is simply that there isn’t a hiatus, and that a quirk in how we measure global surface temperatures has hidden the continuing temperature rise. Ideally, the ‘global average temperature’ should incorporate measurements conducted at a representative variety of places around the world. But some locations are hard to reach. Hence, in practice, temperature records do not encompass the entire globe. In particular, temperature measurements near the poles are scarce – no one really wants to stand in the Arctic year-round with a thermometer! Therefore, global temperature measurements often exclude the high latitudes.
Recently, however, accelerated ice melt and satellite measurements indicate that high latitudes are warming faster than other places on Earth. Those measurements still aren’t included in global temperature trends because they are less precise than surface-based measurements, although they can give an indication of warming rates and show amplification of Arctic warming. Ignoring polar temperature changes when calculating global trends could downplay the problem: the earth could really have been warming the entire time .
The Jury’s Out
Several theories explain the hiatus in global temperature change, but there is no consensus as to which one is correct. To definitively understand the cause of the apparent pause in warming, a better understanding of how energy flows in the Earth system as well as the total energy imbalance and climate forcing is essential. We must improve measurements of energy movement between the atmosphere, ocean, land, and ice using long-term, global monitoring systems. Current records of those systems that we do have in place are frequently too short and too sparse to fully answer the questions surrounding the hiatus.
It is possible that the hiatus may only be completely understood in retrospect, once we’ve examined temperature trends over the next few years. Even so, the hiatus has served to highlight aspects of the climate system we don’t fully understand, forcing the science community to confront difficult questions and highlighting what we don’t fully understand about energy flow in the Earth’s different systems.
Lauren Kuntz is a graduate student in the Department of Earth and Planetary Sciences.
- Tollefson, J. Climate change: The case of the missing heat. Nature 2014. 505: 276-278.
- Trenberth, K. E. and Fasullo, J. T. An apparent hiatus in global warming? Earth’s Future 2013.
- Gillis, J. and Chang K. Scientists Warn of Rising Oceans From Polar Melt. New York Times 2014.
- Trenberth, K. E. and Fasullo, J. T. Track earth’s energy: From El Niño to global warming. Surveys in Geophysics 2011. 33: 413-426.
- Intergovernmental Panel on Climate Change. Climate change 2013: The physical science basis. Cambridge University Press, New York, 2013.
- Meehl, G. A. and Hu, A. Externally forced and internally generated decadal climate variability associated with the interdecadal pacific oscillation. Journal of Climate 2013. 26: 7298-7310.
- Hansen, J. et al. Earth’s energy imbalance and implications. Atmospheric Chemistry and Physics 2011. 11: 13421-13449.
- Cowtan, K. and Way, R. Coverage bias in the HadCRUT4 temperature series and its impact on recent temperature trends. Quarterly Journal of the Royal Meteorological Society 2014.
- NASA Earth Observatory. Global Warming. Retrieved from: http://earthobservatory.nasa.gov/Features/GlobalWarming/page2.php