by Judith Curry
Is global warming slowing down? – David Appell
David Appell has written an extensive article on the pause over at YaleClimateMediaForum, entitled Whither global warming? Has global warming slowed down? The whole article is well worth a read, here I excerpt some quotes from scientists that were interviewed:
These increases are certainly less than the warming rates of the 1980s and first half of the 1990s of about 0.15 to 0.20 C (.27 and .36 F respectively) and per decade. The earlier period may have provided an unrealistic view of the global warming signal, says Kevin Trenberth, climate scientist with the National Center for Atmospheric Research in Boulder, Co.
“One of the things emerging from several lines is that the IPCC has not paid enough attention to natural variability, on several time scales,” he says, especially El Niños and La Niñas, the Pacific Ocean phenomena that are not yet captured by climate models, and the longer term Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) which have cycle lengths of about 60 years.
From about 1975, when global warming resumed sharply, until the 1997-98 El Niño, the PDO was in its positive, warm phase, and heat did not penetrate as deeply into the ocean. The PDO has since changed to its negative, cooler phase.
“It was a time when natural variability and global warming were going in the same direction, so it was much easier to find global warming,” Trenberth says. “Now the PDO has gone in the other direction, so some counter-effects are masking some of the global warming manifestations right at the surface.”
Giving support to their finding is a forthcoming “reanalysis” by Magdalena Balmaseda and Erland Källén of the European Centre for Medium Range Weather Forecasts in the U.K., and Trenberth. Their research, by combining several sources of data with climate models, finds a sharp increase in ocean heating over the past decade, beginning shortly after the 1997-98 El Niño. “In the last decade, about 30 percent of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend.”
In fact, their reanalysis finds that the total of all oceans actually lost heat during the 1990s, at a rate of about -0.26 Watts per square meter of ocean surface area. By contrast, the ocean gained about 1.19 Watts per square meter in the first decade of the 21st century, most in the top 700 meters. That gain, Trenberth says, is associated “with changes in the winds and changes in the ocean currents that are associated with a particular PDO pattern that has dominated in the 2000s.”
So it’s not surprising that there was a significant warming of the surface during the 1990s, but not over the past decade. This recent, large increase in ocean heat content is the best sign that the Earth is still undergoing an energy imbalance caused by an enhanced greenhouse effect.
About 90 percent of this extra energy goes into the oceans. But meteorologist Roger Pielke Sr. of the University of Colorado in Boulder says he would like to understand why more heat is going into the deep ocean. “Until we understand how this fundamental shift in the climate system occurred,” says Pielke, “and if this change in vertical heat transfer really happened, and is not just due to the different areal coverage and data quality in the earlier years, we have a large gap in our understanding of the climate system.”
These large changes in ocean content reveal that the Earth’s surface is not a great place to look for a planetary energy imbalance. “This means this heat is not being sampled by the global average surface temperature trend,” he says. “Since that metric is being used as the icon to report to policymakers on climate change, it illustrates a defect in using the two-dimensional field of surface temperature to diagnose global warming.”
James Hansen, just retired from NASA, wrote recently:
The rapid growth of fossil fuel CO2 emissions in the past decade is mainly from increased coal use…mostly in China with little control of aerosol emissions. It is thus likely that there has been an increase in the negative (cooling) climate forcing by aerosols in the past decade, as suggested by regional aerosols measurements in the Far East, but until proper global aerosol monitoring is initiated, as discussed below, the aerosol portion of the amplified Faustian bargain remains largely unquantified.
However, a recent study by Daniel Murphy of the Earth System Research Laboratory at NOAA, in Boulder, Colorado, found surprisingly little net change in aerosol forcing over the past decade. As air pollution shifted from the northern latitudes of the U.S. and Europe towards the equator in China and India, competing effects largely cancelled one another out — there is more sunlight nearer the equator, but its effect on aerosols is undone by its steeper angle, which means both that it travels through a shorter path in the atmosphere (so has less opportunity to scatter off aerosol particles) and less of its scattering is upward.
Murphy actually found that in the past decade aerosol concentrations have increased the most in the Middle East at about 20 degrees North latitude, perhaps because of dust. Aerosol concentrations decreased around 40 degrees North and around 40 degrees South, with the latter probably brought about by winds that scatter sea salts.
He cautions that his result applies only to aerosol’s “direct effect” — its scattering of sunlight — and not to its many “indirect effects,” such as the function aerosols serve as condensation sites for cloud formation. (The effects are roughly comparable in magnitude.)
“The message is simple,” he says. “For the direct effect, it matters more how much total aerosols there are than where you put them around the Earth.”
“Our expectation has never been that each year would be inexorably warmer than the previous year,” saysBen Santer, a climate modeler at Lawrence Livermore National Laboratory.
It’s simply scientifically incorrect, he says, to attribute the divergence of climate model projections and observations to an overestimation of the climate sensitivity. Santer says he sees several explanations of why climate model projections of surface warming may be differing from actual observations in the past decade or so.
“It’s certainly the case that we got some of the forcings wrong,” he says of the factors that specify the influence of any particular component of the atmosphere. “It’s likely we underestimated the true volcanic aerosol forcing, and may have underestimated the cooling effect of stratospheric ozone depletion.”
The bottom line, Santer says, is “there are multiple, not mutually exclusive interpretations of modeled versus observed differences, and claiming that there is only one explanation is not scientifically accurate.”
“We study the signal. If others want to study the noise, let them.”
Nor is it clear that recent surface trends are particularly unusual. “The term ‘hiatus’ is premature,” says planetary climatologist Raymond Pierrehumbert of the University of Chicago. “Maybe with another 10 years of data you’d say that’s something that needs explanation here.”
Pierrehumbert notes that the increase in carbon dioxide’s radiative forcing over any one decade is about one-fourth of a Watt per meter-squared, so if climate sensitivity is 2 C, the expected warming is only about 0.13 C (forcing increase divided by sensitivity). That can easily be swamped by natural fluctuations of 0.2 to 0.3 C from an El Niño or La Niña, and fluctuations from longer ocean cycles.
And, he says, “There’s really nothing in this that changes our estimates of climate sensitivity.” Calculation of that all-important number from the 20th century record is not possible, because the aerosol forcing is not well known, nor are the data for ocean warming up to the task.
“Any estimate of sensitivity requires all of the record and not just the last 20 years of it,” Pierrehumbert says. “The smaller the piece of it you take, the less certainty you have in your result.”
Nonetheless, he agrees that earlier warming may have been deceiving.
“I think it’s true that some rather sloppy discussion of the rapid warming from the 20th century has given people unrealistic expectations about the future course of warming.”
All the same, the warming effect of carbon dioxide is far down his list of topics that need further examination.
“Why would anyone seriously question greenhouse gases?” he asks. “They absolutely have a radiative effect, and no serious scientist thinks climate sensitivity could be much lower than 2 degrees Celsius based on the balance of the evidence.”
JC comment: A nice job on this article by David Appell. Recall all the flack I took last year for talking about the ‘pause’? The money quote from this article is Pierrehumbert’s: “I think it’s true that some rather sloppy discussion of the rapid warming from the 20th century has given people unrealistic expectations about the future course of warming.” I wonder how long it will be before these scientists take seriously the possibility that sensitivity could be lower than 2C.