Saturday, February 8, 2014

Global warming continues and won’t be stopped by wishful thinking


Global warming has continued rapidly in recent years, mostly accumulating in the oceans. Yet there remains a pervasive myth that it has somehow magically stopped.
Most recently displaying this confusion was Georgia Tech climate scientist Judith Curry, who testified about climate change before a U.S. Senate committee. Curry's testimony was riddled with mistakes and misleading arguments, the worst of which involved disputing that climate scientists are more confident in human - caused global warming than they were six years ago.
Curry based this argument in large part on the supposed global warming 'pause', which is itself a fictional creation. While the warming of average global surface temperatures has slowed (though not nearly as much as previously believed), the overall amount of heat accumulated by the global climate has not, with over 90 percent being absorbed by the oceans, and 30 percent of that in the deep oceans (below 700 meters) over the past 15 years.
A few days after her Senate testimony, Curry took to her blog to dispute these data, essentially arguing that the amount of heat absorbed by the oceans has also 'paused', which would then support her arguments. However, in evaluating the ocean heat content data and scientific literature, Curry made a number of mistakes.
First, she claimed that the only evidence of buildup of heat in the deep oceans comes from reanalysis products. A 'reanalysis' is a climate, ocean, or weather model simulation of the past that incorporates data from historical observations. However, in reality, the increase in deep ocean heat accumulation is a robust result also observed in data sets that do not include reanalysis. 

Ocean temperature
In fact in a paper my colleagues and I published in 2012, we showed that according to the observational ocean temperature data compiled by the National Oceanographic Data Center, about 30 percent of global warming has gone into the deep oceans since the year 2000 – the same result as in subsequent studies using reanalysis data. On top of that, the new paper that Curry referenced in her blog post by scientists from the National Oceanic and Atmospheric Administration (NOAA) found that even more heat has built up in the deep oceans – over 40 percent of overall global warming since 2004.
So what caused Curry to claim that global and ocean warming had 'paused' since 2003? In her blog post, she tried to do science by eyeball. Scientists use statistics because our senses can introduce bias and deceive us. Instead, Curry looked at the following figure from the NOAA paper and decided it looked like ocean warming had stopped in 2003.
The NOAA scientists had actually statistically evaluated data, and found that between 2004 and 2011, the oceans (to a depth of 1,800 meters) had absorbed heat at a rate equivalent of 4.5 Hiroshima atomic bomb detonations per second, according to measurements from instruments on Argo buoys.
In addition to these direct ocean temperature measurements, we also know from satellite observations that the planet is accumulating heat owing to a global energy imbalance. Due to the increased greenhouse effect 'trapping' more heat, there's more incoming than outgoing energy at the top of the Earth's atmosphere. The laws of physics tell us that energy has to go somewhere, which pokes a giant hole in the global warming 'pause' wishful thinking. 
Because they absorb over 90 percent of that energy, we fully expect the oceans to do exactly what they're doing – accumulate a whole lot of heat. If global warming were to 'pause', it would require an explanation of where the energy from the global imbalance measured by satellites is going. 
A new study led by Kevin Trenberth notes that the amount of heat accumulating in the oceans and the rest of the global climate is generally consistent with the global energy imbalance measured by satellites.
There's also the issue of sea level rise, whose main contributors are melting glaciers and ice sheets, and thermal expansion (water expanding as it warms). 

Climate scientists
Climate scientists have been able to close the sea level 'budget' by accounting for the various factors that are causing average global sea levels to rise at the measured rate of about 3.2 millimeters per year since 1992 (when satellite altimeters were launched into space to truly measure global sea level). The warming oceans account for about 40 percent of that rate of sea level rise over the past two decades. If the oceans weren't continuing to accumulate heat, sea levels would not be rising nearly as fast.
Curry also makes the mistake of arguing that the rate of sea level rise during 1930–1950 was similar to that in recent years (most studies estimate that it was lower), which she implies tells us something about the current rise in ocean heat content. This argument is a non sequitur – the conclusion doesn't follow from the premise of the argument. Yes, global warming events have occurred naturally in the past, and sea level rose as a consequence, but that doesn't tell us anything about the causes of the current global warming. This argument is akin to seeing a dead body with a knife sticking out the back and arguing that it must have been a natural death because people have died naturally in the past. 
The bottom line is that all available ocean heat content data show that the oceans and global climate continue to build up heat at a rapid pace, consistent with the global energy imbalance observed by satellites. In recent years, much of that heat has accumulated in the deep oceans. While the rate of increase of global surface temperatures in recent years has slowed in large part due to the more efficient heat transfer to the deep oceans, that can't last forever.
When that trend reverses (likely within the next decade,) we'll experience an acceleration of warming at the Earth's surface. Global warming has only slowed at the surface, not in the overall global climate, and only temporarily. Arguments to the contrary are based on wishful thinking and allowing one's biased senses to be deceived. 
The problem is that we're wasting time hoping that the temporary slowing of rising surface temperatures means we no longer have to worry about global warming, when there's simply no evidence that's the case. The oceans have given us at the surface a temporary reprieve, and we continue to waste it by failing to take serious steps to address the underlying problem; a problem that wishful thinking can't solve.

No comments:

Post a Comment