Keep burning fossil fuels at current rates and you can kiss nature’s influence over temperature good-bye. That’s the conclusion of two recent scientific studies.
Humans are forcing heat trapping substances into the atmosphere at a terrifying pace. We pump out more than 10 billion tons of carbon on the back of about 50 billion tons of CO2 equivalent hothouse gasses into the atmosphere each year. This massive volume is the upshot of an inexorably rising emission starting during the 19th century and continuing to this day. By the end of this century, rates of burning could again increase nearly threefold.
The current, rampant pace of human emission is now at least six times faster than at any time during Earth’s geological past. But on our current path, that rate could exceed 20 times that seen during any of the previous worst hothouse extinction events.
(Current rate of annual carbon emissions in gigatons [black dots] compared to IPCC projected scenarios. Note that current human emissions are on the worst case emissions path. Image source: Global Carbon Project.)
This incredible rate of emission was the key factor in two new studies issued this week investigating the possibility of future hiatuses or ‘pauses’ in global warming due to nature-driven variability (see the studies here and here). And what the studies found was that rampant human burning of fossil fuels removed any possibility for hiatus decades driven by natural variability after 2030.
In essence, we are in the process of shutting down nature’s temperature-related influence entirely.
Understanding Natural Cycling Between Warm and Cool Periods
Natural shifts between atmospheric warm and cool spells appear to be primarily driven by how much heat the oceans uptake or expel.
In the Pacific, this rate of heat uptake is driven by the strength or weakness of the trade winds driving across the equator. During periods in which the trade winds are strong, a great volume of air contacts the surface water and more atmospheric heat is driven into the ocean through down-welling. During periods in which the trade winds are weak, the atmosphere-to-ocean heat transfer shuts down even as warmer waters rise from the depths and spread out across the ocean surface. During these times, the ocean is dumping heat back into the atmosphere.
A similar process happens in the Atlantic where salty, warm surface water down-welling transfers atmospheric heat toward the deep ocean. When that process shuts down, more heat piles up at the ocean surface and bleeds back into the atmosphere.
The first of these processes is called Interdecadal Pacific Oscillation (IPO — which is related to ENSO variations) and is thought to be the primary governor of this global natural variability. The second process, Atlantic Multidecadal Oscillation (AMO), is thought to be the lesser of the two forces.
Past Variability in the Global Temperature Record
Even with large-scale human warming proceeding throughout the 20th Century and into the 21st Century, we can see the effects of this natural variability on the global temperature record. During naturally driven warmer periods human-caused warming advances rapidly. During two of the naturally driven cool periods, human forced warming has a set-back, and, during the third, only seems to briefly slow down.
(Global temperature record as compiled by NASA. Note how warming has traditionally proceeded in a step-like fashion. Image source: NASA GISS.)
Taking a closer look, we find that the time from 1880 to 1910 saw global surface temperatures falling by 0.2 degrees Celsius as ocean heat uptake increased and IPO went negative. From 1910 to 1940, the IPO driver switched into positive. As the oceans disgorged their heat, the first effects of human greenhouse gas heat forcing became evident as global temperatures jumped by 0.45 degrees Celsius over a 30 year period. From 1945 to 1975, IPO again switched into negative, but this time human forcing was in the driver’s seat and temperatures only fell by around 0.06 degrees Celsius. By 1975, temperatures were again on the rise and through 2002, the heat spike rocketed fully 0.6 degrees C upward.
From about 2002 onward, we enter the current ‘hiatus’ period in which atmospheric warming, during a time when we should have seen cooling, has proceeded slowly despite major natural variability factors pushing for cooler atmospheres and warmer oceans.
Reducing Impacts of Natural Variability
The term ‘global warming hiatus,’ however inaccurate, is a new invention. Its use first cropped up over the past couple of years as human greenhouse gas forced warming seemed to slow somewhat from its rampant upward pace through the 1980s and 1990s. This brief pause in atmospheric warming caused some global warming skeptics to declare an end to human-caused heating. An extraordinary claim in the face of highest ever heat-trapping gas emissions.
But what was really happening was that natural variability, which should have been driving the Earth’s atmosphere to cool, was starting to take a back seat.
For two recent studies, mentioned above, found that natural variability driven temperature change has radically fallen even since the 1980s.
The first study, headed by Masahiro Wantanabe, found that, during the 1980s natural variability was responsible for about 47 percent of the observed global temperature change. By the 1990s, this number had fallen to 37 percent. And as an IPO driven switch should have led to cooler temperatures during the 2000s, Wantanabe finds that the effect of natural variability had again plunged to 27 percent.
The cause for the loss of the temperature driving effect of natural variability, according to global climate model runs, is a stunning rate of human greenhouse gas increase. And a related study led by Nicola Maher found that if greenhouse gas emissions by humans kept rising at ever more rapid rates, the natural variability measure is completely overwhelmed by 2030:
The likelihood of future hiatus periods is found to be sensitive to the rate of change of anthropogenic forcing. 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.
Under the worst case emissions scenario — RCP 8.5 — natural variability is completely subsumed by human warming by 2030. Continuing on this track through 2100 means that the human forcing is so strong that even a volcanic eruption on the scale of Krakatau would not be enough to generate a warming hiatus.
For those considering use of solar radiation mitigation through aerosol inject, this point is a very important one to consider. It is a basis for proof that such mitigation eventually radically fails to reduce greenhouse gas heating effects if levels of emissions are not also drawn down.
Sadly, we are currently on the RCP 8.5 track. But, according to the studies, if humans could somehow rapidly reduce greenhouse gas emissions, the effect of natural variability on climate would be at least somewhat preserved.
The discussion of natural variability does not include instances in which human heat forcing produces outcomes outside of natural variability. The most obvious of these would be a large glacial outburst event in which enough water is released from Greenland and West Antarctica to raise seas by 1 meter or more this century. Such an event would have a temporary cooling effect that could result in an unnatural hiatus in warming. Such a human-forced variability was not considered in these global climate model studies, but it is worth considering as the strength of the now rampant human heat forcing continues to increase.
Hat Tip to Colorado Bob