Is there a pause, hiatus, or slowdown?
So here is the basics. If you look at annual global temperatures since 1998, there is no visible rise. 2005 and 2010 are generally regarded as a bit warmer, but our greenhouse gas (GHG) emissions have continued to rise. Where are the steadily rising temperatures? Looking at it this way might suggest (especially if you want to think this way) that global warming has slowed appreciably, or even paused, and that the planet is not really all that sensitive to GHGs after all.
The question if highly complex, and responding to it demonstrates that you can't take the planet's annual temperature and draw firm conclusions about present or future global warming.
For links to a variety of news accounts and analyses of this issues, click here.
For links to scientific papers examining this issue, click here.
But here are two key factors that help us understand what has been going on and why.
1. NATURAL FORCINGS. In addition GHGs, nature has its own way to alter climate, temporarily increasing or decreasing global temperatures. There are three main natural forcings:
(a) fine particulates emitted mainly by volcanoes (called "aerosols"); most of these cool the earth by reflecting the sun's rays back into space.
(b) fluctuations in solar irradiance. Solar energy reaching the planet goes through cycles of greater and lesser intensity--roughly 11 years going up and 11 years going down, and (all else being equal) raise and lower global temperatures accordingly.
(c) changes in the ocean: there are years when the Pacific Ocean is characterized by El Nino, which warms the planet, and years of La Nina, which cools the planet, and some years with neither. There are also longer term fluctuations, in particular the Pacific Decadal Oscillation (PDO). The point is that short-lived changes in the ocean can temporarily increase and decrease global temperatures.
So, we can't just take the planet's temperature and say whether global warming is or is not happening. We have to also consider these natural forcings.
The upshot is this. 1998 was a freak year with the highest El Nino on record, along with solar irradiance near its highest point. The 2000s, on the other hand, were largely a decade in which three things were happening together: declining solar activity, a preponderance of ocean cooling (La Ninas more than El Ninos and a cooling period of the PDO), and some volcanic activity that emitted aerosols that cooled the planet.
Those three things working together temporarily dampened the rise in global temperatures.
AND YET, the decade of the 2000s was considerably warmer than the 1990s, with every year warmer than the average temperature of the 1990s!
In fact, one scientific study recalculated what has been really happening with global warming by removing these natural forcings. What did they find? The planet is continuing to warm. (For a summary of this study, click here.)
2. THE OCEAN. When the planet warms, most of it is actually absorbed by the ocean rather than persisting in the atmosphere or surface of the planet. And what has been happening to ocean temperatures? They have been increasing. (For analyses that discuss this, click here, here, and here.)
So, no, there ain't no pause, despite surface appearances and the wishful thinking of contrarians. We are heating up the planet, and heading toward disaster.
Our choice is how terrible we will allow that disaster will be. We can make the future more livable by dramatically slowing our GHG emissions, or we can let our grandchildren and their grandchildren live a perilous and impoverished world.