The End of Doom (27 page)

The IPCC
Physical Science
report also sets out essentially a “carbon budget” that delineates what quantity of additional greenhouse gases can be emitted without exceeding the 2°C (3.6°F) limit. The IPCC estimates that cumulative carbon dioxide emissions amounting to 5760 gigatonnes (one gigatonne equals one billion metric tons) would mean that there is less than a 33 percent chance of remaining below 2°C. Emitting 4440 gigatonnes raises that to a fifty-fifty chance and emitting even less at 3670 gigatonnes boosts the odds of staying below 2°C rises to 66 percent. Since preindustrial times, humanity has already emitted about 1890 gigatonnes of carbon dioxide. In other words, if the goal is to remain below the 2°C threshold, the IPCC reckons that humanity has already used up about half of its carbon dioxide budget. In November 2014, the IPCC's
Synthesis
Report
concluded, “Scenarios that are
likely
to maintain warming at below 2°C are characterized by a 40% to 70% reduction in GHG emissions by 2050, relative to 2010 levels, and an emissions level near zero or below in 2100.”

Back in his 2007 testimony before a congressional committee, former US vice president and prominent climate change crusader Al Gore sought to summarize the consensus view of climate scientists by declaring, “The science is settled.” US EPA administrator Lisa Jackson reiterated that view in her 2010 testimony to Congress: “The science behind climate change is settled, and human activity is responsible for global warming.” As outlined above, there is indeed broad agreement among climate researchers with regard to a lot of the climate change data and forecasts. Nonetheless, many researchers acknowledge there are some significant unresolved scientific questions that remain to be settled. Further research on these outstanding issues might yield happy results that tend to lower the more dire estimates of future climate change. Let's turn to those questions now.

The Science Is Settled—The Global Temperature Hiatus

“The 5-year running mean of global temperature has been flat for the past decade,” admitted a 2013 global temperature update from the NASA Goddard Institute for Space Studies (GISS) at Columbia University, one of the world's leading climate change research centers. At the time of the GISS update, the hiatus in the increase of global average temperature had actually been going on for nearly sixteen years, ever since 1998. In an August 2014 article, longtime IPCC critic Ross McKitrick applied advanced statistical techniques to global temperature data sets that extended the period during which the global temperature has been essentially flat to encompass the past nineteen years.

The fact that global average temperatures have not significantly increased in the past sixteen to eighteen years is a difficult conundrum for climate researchers to explain. Such a long pause was not predicted by any of the climate computer models relied upon by the IPCC. The 2014
Synthesis
Report
deals with this awkward situation basically by waving it away, asserting that a sixteen-year pause is too short a time from which to draw any conclusions regarding future warming trends.

“Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends,” the IPCC's 2013
Physical Science
report states. Nevertheless, the report asserts this: “The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (
very high confidence
).” What is the observed long-term trend? As previously noted, the trend has been increasing at the rate of 0.12°C per decade. To illustrate how sensitive temperature trends are to starting and ending dates, the IPCC report notes that for fifteen-year periods starting in 1995, 1996, and 1997 the global average temperature trends are 0.13, 0.14, and 0.07°C per decade, respectively. Nevertheless, the IPCC's 2014
Synthesis
Report
observes that the rate of warming over the past fifteen years has been only 0.05°C (0.09°F) per decade, which is considerably lower than the 0.12°C (0.22°F) per decade rate observed since 1951.

So are the models relied upon by the IPCC really all that good at simulating trends in global average temperature? An August 28, 2013, article in the journal
Nature Climate Change
suggests that they are not. In that study Canadian climate researchers pointed out that while global average temperatures rose over the past twenty years at a rate of 0.14
±
0.06°C per decade, thirty-seven of the models relied upon by the IPCC simulated an average rise of 0.30
±
0.02°C per decade. “The observed rate of warming given above is less than half of this simulated rate, and only a few simulations provide warming trends within the range of observational uncertainty,” the authors conclude. It gets worse. For the period after 1998 until 2013, the researchers note, “The observed trend of 0.05
±
0.08°C per decade is more than four times smaller than the average simulated trend of 0.21
±
0.03°C per decade.” The upshot is that, according to this study, the climate models are running four times hotter than the observed temperature trends.

John Christy, a climatologist at the University of Alabama in Huntsville who has long been skeptical of IPCC projections, compared the outputs of seventy-three climate models for the tropical troposphere used by the IPCC in its latest report with satellite and weather balloon temperature trends since 1979 until 2030. “The tropics is so important because that is where models show the clearest and most distinct signal of greenhouse warming—so that is where the comparison should be made (rather than, say, for temperatures in North Dakota),” explains Christy. “Plus, the key cloud and water vapor feedback processes occur in the tropics.” When it comes to simulating the atmospheric temperature trends of the past thirty-five years, Christy found, all of the IPCC models are running hotter than the actual climate. To deflect accusations that he is cherry-picking data, Christy notes that his “comparisons start in 1979, so these are thirty-five-year time series comparisons”—rather longer than the fifteen-year periods whose importance the IPCC report downplays. The private research group Remote Sensing Systems (RSS) also analyzes the satellite temperature data. Like the University of Alabama climatologists RSS reports that “the troposphere has
not
(their emphasis) warmed as fast as almost all climate models predict.” Comparing IPCC climate model simulations to actual temperatures, RSS finds that “after 1998, the observations are likely to be below the simulated values, indicating that the simulations as a whole are predicting too much warming.” Even the IPCC report admits, “Most, though not all, of [the climate models] overestimate the observed warming trend in the tropical troposphere during the satellite period 1979–2012.”

The IPCC
Physical Science
report also concedes that “[a]lmost all [climate model] historical simulations do not reproduce the observed recent warming hiatus.” In fact, the IPCC's technical summary reveals that 111 out of 114 climate models run hotter than the actual observed temperature trend between 1998 and 2012. What accounts for this apparent predictive failure?

The IPCC's
Physical Science
report suggests with “medium confidence” that internal decadal variability is the cause of much of the difference between observations and the simulations. It's fair to say that this pause is something of an embarrassment to many in the climate research community, since the computer models failed to indicate that any such thing could happen. Spurred by the mismatch between computer projections and empirical data, lots of climate scientists have been trying to figure out why the average global temperature has not been increasing significantly.

For example, a 2010 study in
Science
attributed part of the temperature slowdown to decreases in stratospheric water vapor. A September 2011 article in
Nature Climate Change
outlined one of the more popular explanations for the pause. Specifically, the missing heat is supposedly hiding in the deep oceans.

A 2011 article in
Atmospheric Chemistry and Physics
suggested that a prolonged solar minimum combined with atmospheric aerosols left over from volcanic eruptions reduced the amount of heat reaching the surface of the planet.

But as noted previously, the most popular explanation by far for why the atmosphere was not warming even as greenhouse gas concentrations were rising was that the excess heat is hiding in the oceans. Some researchers in March 2014 argued in
Nature Climate Change
that the Pacific Ocean trade winds have speeded up, thus pushing extra greenhouse heat beneath the waves.

In August 2014 researchers at the University of Washington in Seattle and Ocean University of China in Qingdao countered in
Science
that the real reason the atmosphere is not warming is that changes in North Atlantic Ocean circulation are burying the extra warmth. The researchers reported that this process could go on for as long as another twenty years before the ocean begins releasing the stored heat, greatly boosting future rates of warming.

In late August 2014, a team of Japanese climatologists suggested in
Nature Climate Change
that natural variations in Pacific trade winds account for nearly half of the changes in temperature seen over the past three decades. The bad news, according the researchers, is that natural variation is now being overwhelmed by climate change caused by increasing greenhouse gas concentrations in the atmosphere. Consequently, they predict that man-made warming will dominate future temperature trends soon and the hiatus will end.

Also in August, Australian researchers associated with the Pacific trade winds theory report in
Geophysical Research Letters
the results of thirty-one climate models. They find that “under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption.” As it happens, a study published in October 2014 in
Geophysical Research Letters
reports that the effects of volcanic particles on global atmospheric temperatures since 2000 have been underestimated, and they have actually cooled the planet by between 0.05 and 0.12 °C. A January 2015 study by researchers at the Lawrence Livermore National Laboratory in California published in
Geophysical Research Letters
bolstered the findings that sulfuric acid particles from small volcanic eruptions lingering in the boundary between the troposphere and the stratosphere reflect enough sunlight to contribute significantly to the warming hiatus. While volcanic particles in the atmosphere may be masking a warming trend, researchers from the National Center for Atmospheric Research and Duke University in their January 2015 study in
Climate Dynamics
concluded, “We do not find that aerosols exerted a significant global negative forcing over the last decade or so.” In other words, man-made particulate air pollution has not been significantly cooling the planet during the hiatus. This finding contradicts earlier speculations that man-made aerosols might be responsible for a quarter of the discrepancy between model temperature projections and actual temperature trends.

In October 2014, two new papers in
Nature Climate Change
significantly challenged the popular theory that extra heat from greenhouse warming is being buried in the deep ocean. One study analyzed satellite and direct ocean temperature data from 2005 to 2013 and found the ocean abyss below 2,000 meters has not warmed measurably. Basically, satellite measurements between 2005 and 2013 find that sea level has been increasing at rate of 2.78 millimeters per year. Some 0.9 millimeters results from expansion due to warming, and 2.0 millimeters is due to additions of freshwater—for example, melting glaciers. Since 2.9 millimeters is greater than the measured increase of 2.78 millimeters, the researchers concluded that the deep ocean is likely cooling down and thus contracting. In its release describing the work of its researchers, the Jet Propulsion Laboratory noted, “The cold waters of Earth's deep ocean have not warmed measurably since 2005,” thus “leaving unsolved the mystery of why global warming appears to have slowed in recent years.”

In a simultaneously published companion paper, JPL researchers suggest that since the extra heat is not hiding in the deep oceans, it must be instead cached in the upper layers of the southern oceans, where it had gone unnoticed due to spotty measurements over the past thirty-five years. As the JPL release explained, “Using satellite measurements and climate simulations of sea level changes around the world, the new study found the global ocean absorbed far more heat in those 35 years than previously thought—a whopping 24 to 58 percent more than early estimates.” It's always interesting when models find discrepancies in observational data.

The effort to explain or explain away the current warming hiatus is ongoing. In a January 2015
Geophysical Research Letters
article a French research group noted, “The observed global mean surface air temperature (GMST) has not risen over the last 15 years, spurring outbreaks of skepticism regarding the nature of global warming and challenging the upper-range transient response of the current-generation global climate models.” In an attempt to explain the hiatus, the researchers adjust their climate model to try to take into account how surface winds in the Pacific Ocean could be driving heat uptake in the ocean. Even so, they find that their model still overestimates warming compared to actual temperature trends.

Also in January, Duke University climatologists analyzed outputs from thirty-four of the climate models used by the Intergovernmental Panel on Climate Change (IPCC) in its Fifth Assessment Report. They report in the
Journal of Geophysical Research
that the models more or less tracked each other with regard to year-to-year temperature ups and downs but diverged in their explanations for decade-to-decade variability “such as why global mean surface temperatures warmed quickly during the 1980s and 1990s, but have remained relatively stable since then.” Lead author Patrick Brown cautioned, “If you're worried about climate change in 2100, don't over-interpret short-term trends. Don't assume that the reduced rate of global warming over the last 10 years foreshadows what the climate will be like in 50 or 100 years.” On the other hand, Brown also noted, “The inconsistencies we found among the models are a reality check showing we may not know as much as we thought we did.”