No, global warming hasn’t stopped and here’s why

פורסם: 23 ביולי 2013, 11:57 על ידי: Sustainability Org   [ עודכן 23 ביולי 2013, 11:58 ]
By James Temple

In 1998, the global mean temperature was 58.3 degrees Fahrenheit, according to NASA’s Goddard Institute for Space Studies. In 2012, it was 58.2 degrees.

That’s a 0.1 degree decrease. Look, I disproved global warming! Hummers for everyone!

As ridiculous as it sounds, that simplistic analysis is the basis for one of most frequently cited critiques of climate science. Indeed, any time I write about global warming an email rebuttal arguing the globe hasn’t heated in 15 years reliably lands in my inbox.

Those readers are likely taking their talking points from the many professional climate deniers who repeat this inaccuracy as often as possible, including in op-eds in Forbes and the Wall Street Journal.

“Warming ended 15 years ago, and global temperatures have stopped increasing since then, if not actually cooled, even though global CO2 emissions have soared over this period,” wrote Peter Ferrara, director of entitlement and budget police at the Heartland Institute, in a representative piece.

This conclusion isn’t at all surprising from a conservative think tank that routinely goes to great lengths to sow doubts about the science of global warming in the public mind. The problem is that arriving at it requires ignoring everything but the two dots on a chart that, in isolation, seem to make their case.

Let’s start by looking at the data in question.

Global Temp 1998 forward

It becomes immediately obvious that this is a classic case of manipulating statistics to reach a predetermined conclusion, specifically by cherry picking the start date. That red line that deniers are relying on doesn’t actually conform to the shape of that chart.

If you want to know if a climate denier is attempting to mislead you, the first clue is that they’re starting with the year 1998. It was one of the hottest years on record thanks to an unusually strong El Niño.

“The 1998 spike caused by an extraordinary El Nino event has been statistically abused for a long time,” said Reto Ruedy, a research associate at NASA, in an email. “What appeared to be an extraordinary global temperature anomaly 15 years ago is now an expected occurrence and has been — within the margin of error — equaled 8 times since then.”

In fact, he pointed out, the margin for error in these numbers is about 0.1 degree Fahrenheit, so there’s actually no statistical difference between the years 1998 and 2012.

Start your analysis at the year 1999 or strip out the anomalous year of 1998, and suddenly you see a strong warming trend.

But there’s no reason for that. The truth is there’s a huge amount of variability in the climate system, temperatures bounce up and down from year to year. It’s messy stuff. That’s why climate scientists care more about long-term trends, favoring at least 30-year cycles.

So let’s look at that:

Global Temp 1980 forward

There’s simply no questioning the trend line here. But let’s zoom out even further.

Global Temp 1900 forward

Quite a different picture.

“When skeptics or deniers say look at this little graph that shows that temperatures are not warming anymore, they’re misleading or misreading or both,” said Peter Gleick, president of the Pacific Institute, a research institute in Oakland.

Now it is true that some scientists acknowledge a decline in the rate of temperature increases in recent years, what some have dubbed the climate change plateau or slowdown. But nothing about that is particularly reassuring — or gets us off the hook for our skyrocketing greenhouse gas emissions.

First off, consider the conditions we’ve witnessed during this period. Regardless of the rate of increase in temperatures, globally nine of the ten hottest years on record have all occurred since 2000.

“It would be absurd to use the hottest 10 or 15 years on record to argue that we don’t need to worry about the Earth getting even hotter,” said Ken Caldeira of the Carnegie Institution in an email.

We’re already living with the consequences of climate change, including more extreme weather events, melting ice caps, rising sea levels and more.

Caldeira said that one of three things is likely at work in the plateau: radiative forcing from carbon dioxide, which is essentially the amount of energy that the added CO2 prevents from escaping, is less than scientists thought; more energy is being absorbed by the oceans than previously believed; or more energy is escaping into space.

He said additional research is needed to know what’s really underway, but if he were forced to bet, he’d go with the warming ocean. A handful of recent studies also point to the ocean, specifically the deep ocean, as the culprit.

The oceans absorbed about 90 percent of the heat added to the climate system during the last 50 years, according to a study published in May in Geophysical Research Letters. And for some reason, the deep ocean became “much more strongly involved in the heat uptake after 1998,” the report said.

That’s bad news. Warmer oceans alter weather patterns, stir up more powerful storms and threaten all sorts of sea life. And as much attention as melting ice caps get for their role in rising sea levels, the other major cause is warmer water, which simply takes up more space.

Here’s what the temperature picture looks like when you toss ocean heating into the mix, per a chart published earlier this year on Skeptical Science.

Total Heating

Feel reassured now?

“When you look at the bigger picture, warming hasn’t stopped,” Gleick said.

Now some scientists do think there’s a possibility that the climate system is slightly less sensitive to growing carbon dioxide concentrations than previously thought. A report published in May in the journal Nature Geoscience looked at temperatures in the last decade and concluded that “equilibrium climate sensitivity,” might fall into the lower part of earlier ranges.

It’s almost odd to have to point this out, but if so, that is good news for the earth and its species. It could mean a little more time to deal with the still incredibly abrupt changes underway in our climate system.

But no matter what deniers will make of such reports, it’s not evidence that the globe has stopped warming nor that we’ve sidestepped a gigantic problem that demands our immediate attention.

“Global temperatures have been increasing for decades,” Caldeira said. “From a policy perspective, we have no choice but to transform our energy system into one that does not use the atmosphere as a waste dump.”

For more stories about the challenges of climate change and Bay Area thinkers working on cutting-edge ideas to help us survive a warmer world, please read more in our “Taking the Heat” series here. Or view the video below.