We knew that, but how fast? And why haven’t we had a hottest year since 2010?
How much has temperature changed?
NASA has a number of graphs for the US and the world. This one shows more of a temperature increase in the northern hemisphere, where there is more land.
This trend is somewhat easier to see with separate graphs for El Nino, La Nina, and ENSO neutral years.
Credit: Skeptical Science
The temperature increase at the surface has slowed down some. Earth has been warming at less than 0.2°C/decade:
The three major surface temperature data sets (NCDC, GISS, and HadCRU) all show global temperatures have warmed by 0.16 – 0.17°C (0.28 – 0.30°F) per decade since satellite measurements began in 1979.
Surface warming is currently below that rate; over the last decade, temperature increase at the surface has only been 0.081 ± 0.13°C.
The Intergovernmental Panel on Climate Change prediction is that Earth’s surface continues to warm by 0.2°C/decade, to one sig fig, for the next 2 decades, but the range is huge, from slightly negative to more than 0.3°C/decade, depending on where heat is stored.
OK, temperature increase is between a negative amount and 0.3°C this last decade, but why is it at the lower end?
Changes in natural forcings
The sun goes through an 11-year cycle, but there are variations from cycle to cycle. Comparing to other years when solar irradiance is at a maximum, Earth receives about 0.1 watt/square meter less sunlight.
This may not sound like much decrease from 240 W/m2 absorbed normally, except that the change since 1750 has only been 1.6 W/m2.
Then there are volcanoes. Jeff Masters points to
a study published in March 2013 in Geophysical Research Letters found that dust in the stratosphere has increased by 4 – 10% since 2000 due to volcanic eruptions, keeping the level of global warming up to 25% lower than might be expected.
This result was surprising. Previously, it was thought that Pinatubo-sized eruptions could release enough sulfur dioxide to affect climate in the short term, but not small to moderate volcanoes. While the increase in Asian pollution is also cooling Earth, the effect of small to moderate volcanoes has been more important.
Ryan Neely, an atmospheric scientist at National Center for Atmospheric Research (NCAR)
cautions that, while the new study shows the importance of volcanoes on a decadal level, there is a need to learn more about their effects on year-to-year climate variability as well. “Though we show that volcanoes had the most impact in this instance, this has not and may not always be true,” he says.
So are net heat flow and the rate of warming decreasing?
The energy budget of Earth refers to the flow of heat in, from the sun, and out. Heat is reflected, and heat flows out of any hot body.
According to Skeptical Science,
This energy imbalance was very small 40 years ago but has steadily increased to around 0.9 W/m2 over the 2000 to 2005 period, as observed by satellites. Preliminary satellite data indicates the energy imbalance has continued to increase from 2006 to 2008. The net result is that the planet is continuously accumulating heat.
Note that because the change in forcings is 1.6 W/m2, that we are out of balance by 0.9 W/m2 indicates we will continue to warm for some time, assuming that atmospheric levels of greenhouse gases remain constant.
Kevin E. Trenberth, head of the Climate Analysis Section at NCAR, and a lead author for the 2001 and 2007 IPCC Scientific Assessment of Climate Change (Working Group 1), wondered,
with this ever increasing heat, why doesn’t surface temperature continuously rise? The standard answer is “natural variability”. But such a general answer doesn’t explain the actual physical processes involved. If the planet is accumulating heat, the energy must go somewhere. Is it going into melting ice? Is it being sequestered deep in the ocean? Did the 2008 La Niña rearrange the configuration of ocean heat? Is it all of the above?
Now we know that much of the heat is being stored in the ocean at depths below 700 meters (2300 ft):
The preponderance of La Niña events in recent years has caused a large amount of heat from global warming to be transferred to the deep oceans, according to a journal article published earlier this week by Balmaseda et al., “Distinctive climate signals in reanalysis of global ocean heat content”.
Is that good or bad?
The next big El Niño event will be able to liberate some of this stored heat back to the surface, but much of the new deep ocean heat will stay down there for hundreds of years. As far as civilization is concerned, that is a good thing, though the extra heat energy does make ocean waters expand, raising sea levels.
Can we stop it?
In a perspective piece in the March 28, 2013 Science (subscription required), H. Damon Matthews and Susan Solomon (lead author in Working Group 1 of 2007 IPCC report), Irreversible Does Not Mean Unavoidable, say there is confusion:
irreversibility of past changes does not mean that further warming is unavoidable.
The climate responds to increases in atmospheric CO2 concentrations by warming, but this warming is slowed by the long time scale of heat storage in the ocean, which represents the physical climate inertia. There would indeed be unrealized warming associated with current CO2 concentrations, but only if they were held fixed at current levels. If emissions decrease enough, the CO2 levels in the atmosphere can also decrease. This potential for atmospheric CO2 to decrease over time results from inertia in the carbon cycle associated with the slow uptake of anthropogenic CO2 by the ocean. This carbon cycle inertia affects temperature in the opposite direction as the physical climate inertia, and is of approximately the same magnitude.
Because of these equal and opposing effects of physical climate inertia and carbon cycle inertia, there is almost no delayed warming from past CO2 emissions. If emissions were to cease abruptly, global average temperatures would remain roughly constant for many centuries, but they would not increase very much, if at all. Similarly, if emissions were to decrease, temperatures would increase less than they otherwise would have…”
So if we cease to add greenhouse gases to the atmosphere, atmospheric greenhouse gases concentrations would begin to decline, due to ocean, etc uptake. Earth would continue to warm because heat flow in is still larger than heat flow out, but some of this heat would be taken up by the oceans. Over time, Earth’s surface would heat very slowly if at all. It would not cool for a long while. Projections show temperatures continuing to go up because we will continue to emit GHG for some time.
More explanation here.
Satellite measurements find Earth is warming faster than in the 1990s. This is occurring even with a cooler sun and a temporary increase in sulfate particles in the atmosphere, reflecting more heat. Earth’s surface is warming more slowly because with La Ninas, heat is stored in the oceans at depths below 700 meters. (There is still some heat flow not yet accounted for.) When heat is stored in the deep oceans, sea level increases more rapidly, while our climate changes more slowly. Some of this heat will be returned to the atmosphere with the next El Nino.
Reducing, even zeroing out, GHG emissions is an excellent idea.