There's a rather common meme among climate contrarians that global warming has stopped. Here's some quotes from a few commentators on io9:
The Earth hasn't on average warmed in a decade
temps have not risen in at least 16 years and they actually are trending toward cooling
computer models have failed to predict or account for the last 15-17 years of no statistical warming
Whatever degree of warming there has been stopped 15 years ago
temperatures have plateaued over the last 15 years
Globally, it hasn't warmed relative to 1998
The interesting thing about this meme is that it shows some memetic variation that belies its pseudoscientific roots: the warming has stopped somewhere between 10 and 18 years, apparently dependent on whom is asked. To understand how this meme is nonsensical, I have to touch on a few pretty basic things first.
What is the greenhouse effect?
The greenhouse effect is a consequence of the Earth's atmosphere. The sun emits large amounts of electromagnetic radiation in a variety of wavelengths, but the large majority of solar radiation comes to Earth in wavelengths of ultraviolet, infrared, and visible. This radiation partially passes through the Earth's atmosphere: some wavelengths are reflected back into space, and some are absorbed by molecules within the atmosphere. What remains travels to the surface of the Earth, providing some heat through partial absorption, but the majority of the radiation that reaches the Earth's surface bounces off the Earth's surface. In the atmosphere, this radiation is partially absorbed and re-emitted by greenhouse gases, which generally 'bounce' the radiation either into space or back to the Earth's surface as infrared.
What is global warming?
Global warming is a trend of climate change wherein parts of the Earth show a long-term increase in temperature. When I say parts of the Earth, I am pretending that the Earth is composed of five fluid compartments. The first of these is the thing we all interact with (unless we're in space), the troposphere. Above the troposphere is the stratosphere, and above that is what we can simply label the upper atmosphere. Returning to sea level, we find the upper oceans, which interact heavily with the troposphere, and below them the deeper oceans, which do not.
Global warming can occur in all five of these compartments at the same time. Generally, any form of warming that is based on stimuli that are external to the Earth would cause warming throughout the atmosphere and into the oceans. Modern global warming causes observable warming in the troposphere, in both the shallow and deep oceans, and ... cooling in the stratosphere.
This stratospheric cooling has two main causes, both anthropogenic. One is that the decline of the ozone layer allows for less heat absorption by ozone. The other is greenhouse gas emissions: as heat is trapped preferentially within the troposphere, less is allowed to reach the stratosphere. Stratospheric cooling, in combination with tropospheric warming, is strong evidence that modern global warming is attributable to changes within the Earth's fluid compartments, not changes from solar or cosmic sources.
How could global warming stop?
In order for global warming to stop, long-term (let's say 14±4 years) changes in temperature within these five fluid compartments would have to show no change or negative change. A lack of warming in one of the compartments, uncoupled with a lack of warming in the other compartments, would not be global warming, it would be regional warming.
Has global warming stopped?
Definitely nope. Every analysis of heat concentration in the upper and deeper oceans shows that there's at least a ~40 year trend towards increased heat concentration. The only way to claim that global warming has stopped is to (purposely or accidentally) ignore the compartments of the Earth that have a heat capacity a thousand times greater than the troposphere.
Has global surface temperatures warming stopped?
Here's the trend of 18 years of surface temperatures according to 4 different datasources, all four of which show warming.
Here's the trend of 10 years of surface temperatures according to the same datasources. Two of these show warming, two of these show cooling.
Intervening years are similarly confused. Trends since 1997, 1998, 2000, and 2003 shows three increases and a decrease. Trends since 1999 show four increases. Trends since 2001 show two increases and two decreases. Trends since 2002 show three decreases and one increase. Of 36 combinations of years and datasource, 25 show warming trends and 11 show cooling trends. If there's two trends supporting warming for every one trend which supports not-warming, it's not realistic to report that as "there is no warming."
What to say to people who are confused about this?
1. There is no strong evidence for a warming pause in surface temperatures. Go look at the data and see that there's more evidence for continued surface warming than there is for paused surface warming.
2. The surface temperature pause, if it existed, would not falsify global warming, because oceans continue to heat.
3. The where of global warming (within oceans and the troposphere, but not the stratosphere) is strong evidence that modern global warming is primarily caused by factors that are internal to, not external to, Earth's oceans and atmosphere.
4. There's some evidence that the slowdown of surface warming is partially because of decreased human emissions of CFCs. If this evidence is not wrong, then an intergovernmental agreement to limit industrial emissions helped slow down global warming and did not cause financial ruin.
Ms. Newitz wrote an article recently that discussed the non-existent pause in global surface warming. It's a great article explaining why some of the global temperature datasources have limitations that skew data away from warming, but she leads the article with the statement
Global temperatures haven't gone up in the last few years.
This isn't true, and it's really unfortunate to see someone, in an otherwise well-written article, jump onto a false memetic bandwagon.
Comments at the start of the article come from a few contrarians in the io9 commentary community. Thanks for the material, folks.