Twelve years ago, Paul Crutzen, a Nobel laureate and atmospheric chemist, coined the term "Anthropocene" as shorthand, an argument wrapped in a word. Geology had long relegated humanity to the sidelines, but in recent history, the human fingerprint on the Earth had grown too deep to be ignored, he said. We had created our own geological time. The world had left the Holocene behind and entered an epoch of humanity.
In other words, humanity has now so fucked up the world that it is human actions which largely determine basic environmental factors like the climate and the oceans. This point becomes clearer when we consider this chart, which has led to the type of confused discussion which humans are prone to. (And read here to untangle some of the latest confusion.)
Figure 1 from Global Temperature Update Through 2012 by J. Hansen, M. Sato, R. Ruedy (NASA GISS). "Global surface temperature anomalies relative to 1951-1980. The El Nino index is based on the detrended temperature in the Nino 3.4 area in the eastern tropical Pacific. Green triangles mark the times of volcanic eruptions that produced an extensive stratospheric aerosol layer. Blue vertical bars are estimates of the 95% confidence interval for comparisons of nearby years."
And here is the bone of contention, from that NASA document.
Global Warming Standstill — The 5-year running mean of global temperature [red line in Figure 1] has been flat for the past decade. It should be noted that the "standstill" temperature is at a much higher level than existed at any year in the prior decade except for the single year 1998, which had the strongest El Niño of the century.
However, the standstill has led to a widespread assertion that "global warming has stopped". Examination of this matter requires consideration of the principal climate forcing mechanisms that can drive climate change and the effects of stochastic (unforced) climate variability.
I can not possibly explain what's going on in a single blog post, so consider this the first in a series on the "Global Warming Standstill" we see in the 5-year running mean for global temperature. There are three main factors which have caused this "standstill" (see Kaufmann, et. al., 2011).
- Decreased solar insolation due to natural cycles of the Sun
- Global cooling during the "La Nina" phases of ENSO (El Niño/Southern Oscillation), which is a large natural variability in the southern Pacific Ocean affecting global temperatures
- The cooling caused by sulfate aerosols in the atmosphere, which are produced when coal is burned or, more importantly, when big volcanoes erupt—look at the green triangles in Figure 1 above.
It is not as though the heat-trapping properties of greenhouse gases (CO2, CH4, etc.) have magically disappeared, and although there is room for reasonable argument about what has been happening over the last 12 years, reasonable argument is a rare commodity on Planet Stupid (see the links above). It is not as though the alarming warming trend in the Arctic has not continued apace.
It is factor #3 which suggests that humanity has now entered the Anthropocene, which is probably the least significant of the three climate factors listed. Nonetheless, the cooling effect of sulfate aerosols can not be ignored, as NOAA's Susan Solomon argued in 2011.
The latest research, carried out by Susan Solomon of the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colorado, and colleagues, focuses on this "background" level of aerosols and takes advantage of the fact that there have been no major eruptions, which obscure the background, since that of Mount Pinatubo in 1991.
Solomon and co-workers took data from a number of sources, including ground- and laser-based measurements recorded at the Mauna Loa volcano in Hawaii, as well as satellite observations. The satellite data reveal that stratospheric aerosols increased by about 7% a year between 2000 and 2010. This implies a change in the Earth's radiative forcing – a measure of the imbalance between the Earth's incoming and outgoing energy – of about –0.10 Wm–2. As the researchers point out, this compares to an annual change in atmospheric carbon dioxide of about 0.5% a year and an overall increase in radiative forcing over the last decade of +0.28 Wm–2, making the contribution of the aerosols small but "significant".
[My note: Wm–2 is a measure of radiative forcing.]
To calculate the effect of this negative forcing on temperatures, Solomon's group used the Bern 2.5cc climate model. This is not as complex as the "general circulation" models used to compute the overall climate but is, they say, better for investigating small temperature changes that might otherwise be hidden by the larger climate trends. They found that the stratospheric aerosols reduced warming over about the last decade by 0.07 °C, or 20%. This is when compared to a scenario with no aerosols, which is the assumption made by many models. They also calculated, based on more limited balloon data, that the presence of background aerosols reduced warming by about –0.05 °C between 1960 and 2000.
Flattening temperature rise
One of Solomon's colleagues, John Daniel of the Earth System Research Laboratory, says that the group was motivated to model this cooling effect by a recent flattening out of the global temperature rise, although he cautions that this rise appears to be quite short term. As previously highlighted by NASA's James Hansen, who was not involved in the current research, the flattening is present when temperatures are averaged out over periods of five years, but not when the average is taken across the whole of the 2000 to 2010 decade.
Daniel, however, emphasizes that whether or not there has been a real flattening of late is essentially irrelevant to the conclusion of the latest paper – that background levels of stratospheric aerosols must be taken into consideration in future climate projections. The researchers say that any climate models that neglect changes to these levels relative to the year 2000 are likely to overestimate warming if aerosol concentrations remain constant or continue to increase. On the other hand, they say, if concentrations were instead to drop back down to the levels last seen in 1960 then global average temperatures will be around 0.06 °C higher by 2020 than they would otherwise have been.
As you know, the Chinese (among others) have been burning coal over the last decade like there's no tomorrow (and if you wait long enough, there won't be one) . Aerosol levels in the atmosphere only account for 20% of the observed "cooling" over the last decade—actually a flattening of the 5-year running mean—but, paradoxically, burning coal produces sulfate aerosol pollution (a weak "negative" climate forcing) which cancels out a small part of the warming caused by the CO2 pollution which is the main byproduct of burning it (a strong "positive" climate forcing).
It is this kind of human-created confusion which caused Paul Crutzen to coin the term "Anthropocene" twelve years ago. I will have more to say about this in the future.