March 2, 2010

Most of the Observed Warming since the Mid-20th Century Likely Not from Human GHG Emissions?

Filed under: Temperature History

A few weeks ago, over at the blog MasterResource.org, WCR’s Chip Knappenberger took a look at just how confident one should be regarding the amount of warming that anthropogenic greenhouse gas (GHG) emissions have caused since the mid-20th century.

The IPCC claims that it is “very likely” that “most” of the warming since then has been the result of human GHG emissions. In IPCC parlance, “very likely” means with a greater than 90% likelihood. The EPA parrots the IPCC’s claim in the Technical Support Document for their Endangerment Finding (TSD, p. 2):

Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG [greenhouse gas] concentrations.

But, in his MasterResource.org article, Knappenberger shows that this statement is not supported by recent findings in the scientific literature—findings that have appeared in the literature subsequent to the publication of the IPCC’s statement. He concluded that the IPCC’s statement—especially the likelihood designation—should be re-evaluated in light of what we know now.

In some sense, however, Knappenberger’s analysis did not go far enough. While he used middle-of-the-road estimates for the warming influence of some non-GHG factors, in some cases he was being too conservative—like when it comes to the non-climatic influences on local thermometers—and, further, he failed to include a potential impact from solar changes.

So here, we take Knappenberger’s analysis a bit further, and show that it is easy to demonstrate, using the contents of the peer-reviewed scientific literature, that anthropogenic GHG emissions could be responsible for less than one-third of the warming in the extant global temperature records.

Let’s start from the beginning.

Figure 1 shows the temperature history from the U.K. Hadley Center from 1950 to 2009. There is a total rise (based on the best-fit linear regression) of 0.702°C during this time.


Figure 1. Annual global average temperature history from 1950 to 2009 (source: U.K. Hadley Center).

Soon after the IPCC Fourth Assessment Report was released, a paper was published in the journal Nature, by David Thompson and colleagues which documented a cold bias (resulting from changing sea surface temperature measuring techniques) contained in the global temperature records originating centered in the early 1940s and extending into the mid-1960s. This error was not related to GHG emissions, and a reasonable stab at correcting for it (an official correction has not been released) reduces the warming from 1950 to 2009 to 0.552°C—or 79% of the original “observed” warming (Figure 2).


Figure 2. Annual global average temperature history from 1950 to 2009 (source: U.K. Hadley Center) and adjusted annual global average temperature to remove SST errors (Thompson et al., 2008).

But some of the remaining warming is caused by changes to the temperature observing network from things such as local land-use changes, urbanization influences, changes in thermometers, degradation of station quality, etc. The keepers of the Hadley Center temperature dataset maintain that the net influence of such changes is minimal (~0.002°C/decade), but other researchers aren’t so optimistic (e.g. Pielke Sr. et al., 2007; Klotzbach et al., 2009). In work aimed at quantifying the non-climatic influence on the land-based temperature record, WCR’s Patrick Michaels and colleague Ross McKitrick found that as much as one-half of the warming observed over land areas since 1980 was caused by non-climatic factors. There is little reason to think that the situation was much different in the three decades prior. So, factoring out the non-climatic contamination of the land temperature data (remembering that 70% of the world is covered by oceans) as identified by McKitrick and Michaels, reduces the total warming from 1950 to 2009 to 0.468°C or 67% of the original “observed” warming.


Figure 3. Annual global average temperature history from 1950 to 2009 (source: U.K. Hadley Center) and adjusted annual global average temperature to remove SST errors (Thompson et al., 2008) and non-climatic influences (McKitrick and Michaels, 2007).

Another source of warming potentially not related to human GHG emissions was recently identified by Solomon et al. in a publication in Science magazine. Solomon et al. found that variations in the amount of water vapor is the earth’s upper atmosphere (stratosphere) have been responsible for some of the surface temperature trends observed in recent decades. Solomon et al. tend to think the water vapor variations are not closely linked to anthropogenic GHG emissions. Thus, we need to remove them from our tabulation as well. Solomon et al. find that stratospheric water vapor increase has been responsible for about 15% of the warming since 1980. Removing this potential non-GHG influence drops the total warming from 1950-2009 to 0.408°C—or 58% of the current “observed” warming.


Figure 4. Annual global average temperature history from 1950 to 2009 (source: U.K. Hadley Center) and adjusted annual global average temperature to remove SST errors (Thompson et al., 2008), non-climatic influences (McKitrick and Michaels, 2007) and the influence of stratospheric water vapor increases (Solomon et al., 2010).

Another potential source of non-GHG warming is black carbon aerosols, a.k.a. soot. Black carbon warms the earth by dirtying both the atmosphere and the surface—in particular snowy surfaces. The IPCC does include black carbon emissions as a positive climate forcing, but, apparently not to the degree that observations suggest it should be. In a 2008 review of published research on the topic, Ramanathan and Carmichael concluded that black carbon is responsible for about 25% of the positive climate forcing since the pre-industrial period. Since positive climate forcing directly translates into global warming, the implication of Ramanathan and Carmichael’s conclusion is that about one-quarter of the observed warming is from black carbon. So, we must factor out this non-GHG effect from the remaining warming. Now we are left with a total change of 0.306°C—or 44% of the original value.


Figure 5. Annual global average temperature history from 1950 to 2009 (source: U.K. Hadley Center) and adjusted annual global average temperature to remove SST errors (Thompson et al., 2008), non-climatic influences (McKitrick and Michaels, 2007), the influence of stratospheric water vapor increases (Solomon et al., 2010) and the influence of black carbon aerosols (Ramanathan and Carmichael, 2009).

And let’s not leave out the non-GHG influence of variations in the output of the sun. If the solar output varies, so too will the earth’s temperature. Duke University’s Nicola Scafetta has been studying the potential influence of solar variability on the earth’s recent temperature trends for several years now. His most recent calculations were published last year in the Journal of Atmospheric and Solar-Terrestrial Physics. He concluded that it is impossible to precisely nail down the solar influence in recent decades because of uncertainties induced from trying to stitch together non-continuous measurements made from several different satellite-borne instruments. However, Scafetta did provide three plausible solar output histories since 1950 and calculated the potential impact of each. He found that solar variability since 1950 could have contributed anywhere from virtually no warming to upwards of 65% of the observed warming. In his middle scenario, Scafetta found that increases in the solar output since 1950 could have been responsible for about 33% of the observed warming since then. If we factor this non-GHG warming out, we are left with a total temperature rise of 0.204°C from 1950-2009—or just 29% of the original “observed” warming.


Figure 6. Annual global average temperature history from 1950 to 2009 (source: U.K. Hadley Center) and adjusted annual global average temperature to remove SST errors (Thompson et al., 2008), non-climatic influences (McKitrick and Michaels, 2007), the influence of stratospheric water vapor increases (Solomon et al., 2010), the influence of black carbon aerosols (Ramanathan and Carmichael, 2009) and the influence of solar variability (Scafetta, 2009).

So there you have it. Using findings that have appeared in the mainstream peer-reviewed scientific literature, it is straightforward to build a case that less than one-third of the observed warming since the mid-20th century is from human GHG emissions.

Now, whether or not all of these impacts are as large as they have been reported, or whether they are totally independent of each other (as we have assumed), may or may not bear out as more research in done in the future. But, more research may just as well determine that these effects may be larger than reported, or may identify yet other warming influences. We can’t say for sure—nor can anyone else.

But one thing that we can say for sure is that the IPCC’s statement that “[m]ost of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic GHG [greenhouse gas] concentrations” is badly in need of reassessment. The current literature shows that it cannot be justified.

References:

Klotzbach, P. J., et al., 2009. An alternative explanation for differential temperature trends at the surface and in the Lower Troposphere, Journal of Geophysical Research, 114, 10.1029/2009JD011841.

McKitrick, R. R., and P. J. Michaels, 2007. Quantifying the influence of anthropogenic surface processes inhomogeneities on gridded global climate data. Journal of Geophysical Research, 112, D24S09, doi:10.1029/2007JD008465.

Pielke Sr., R. A. et al., 2007. Unresolved issues with the assessment of multidecadal global land temperature trends. Journal of Geophysical Research, 112, D24S08, doi:10.1029/2006JD008229.

Ramanathan V., and G. Carmichael, 2009. Global and regional climate changes due to black carbon. Nature GeoScience, 1, 221-227.

Scafetta, N., 2009. Empirical analysis of the solar contribution to global mean air surface temperature change. Journal of Atmospheric and Solar-Terrestrial Physics, 71, 1916-1923.

Solomon, S., et al. 2010. Contributions of stratospheric water vapor to decadal changes in the rate of global warming. Science, published on-line January 28, 2010.

Thompson, D., et al., 2008. A large discontinuity in the mid-twentieth century in observed global-mean surface temperature. Nature, 453, 646-649




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