Sorry, Deniers, Study of “True Global Warming Signal” Finds “Remarkably Steady” Rate of Manmade Warming Since 1979
"Sorry, Deniers, Study of “True Global Warming Signal” Finds “Remarkably Steady” Rate of Manmade Warming Since 1979"
We knew that even the Koch-funded Berkeley study found recent surface warming “on the high end” and speeding up. And scientists have long known that the overwhelming majority of human-caused warming was expected to go into the oceans, which just keeps heating up (see charts at the end).
Now a new study goes one step further and removes the “noise” of natural climate variability from the temperature record to reveal the true global warming signal. That noise is “the estimated impact of known factors on short-term temperature variations (El Niño/southern oscillation, volcanic aerosols and solar variability).”
The deniers have been making as much noise as they can about the “noise” over the years — since it has obscured the rate of warming in the past dozen years, especially compared to a cherry-picked starting point of 1998 (a year whose temperature was boosted by one of those short-term variations, a big El Niño:
Figure 1: The departure from global average temperature since 1979 using the raw data of the 3 surface temperature records (GISS, NCDC, CRU) and the two satellite records (RSS, UAH).
Here’s what happens when you remove the noise and average all 5 temperature data sets:
Figure 2: The “true global warming signal.”
The authors of the study note the “adjusted data show clearly, both visually and when subjected to statistical analysis, that the rate of global warming due to other factors (most likely these are exclusively anthropogenic) has been remarkably steady during the 32 years from 1979 through 2010.” They conclude:
Its unabated increase is powerful evidence that we can expect further temperature increase in the next few decades, emphasizing the urgency of confronting the human influence on climate.
For those who want more analysis, I’m reposting Tamino’s excellent post below:
The Real Global Warming Signal
Many different factors affect Global temperature. Fake “skeptics” like to claim that mainstream climate scientists ignore everything but greenhouse gases like CO2, when in fact it’s mainstream climate scientists who identified those other influences. Natural factors cause temperature fluctuations which make the man-made global warming signal less clear, fluctuations which are often exploited by fake skeptics to suggest that global warming has paused, or slowed down, or isn’t happening at all. A new paper by Foster & Rahmstorf accounts for some of those other factors, and by removing their influence from the temperature record makes the progress of global warming much more clear.
The paper studies the five most often-used global temperature records. Three of them are surface temperature estimates, from NASA GISS, HadCRU, and NCDC, the other two are satellite-based lower-atmosphere estimates from RSS and UAH. These are compared to three factors which are known to affect climate: the el Nino southern oscillation, atmospheric aerosols (mostly from volcanic eruptions), and variations in the output of the sun. The time span studied was from January 1979 through December 2010, for which all five data sets have complete coverage.
Their influence was estimated by multiple regression. In addition to these natural influences the regression also included a linear trend in time, allowing a simultaneous estimate of the rate of global warming as well as the impact of these other factors. Since the natural influences can have a delayed effect on temperature, the regression allowed for a lag between the value of any of the three factors and its impact. Once the effect of the three known factors was estimated, it could be removed from the temperature data to create adjusted temperature data which is mostly (but not completely!) free of their influence.
The raw data — before the natural fluctuations are removed — look like this [see Figure 1].
All five records show similar changes, including an upward trend over the 32 years studied. They also show large fluctuations, more so for the satellite data than the surface data. This has spurred numerous false claims of silly things like “global warming stopped in 1998″ (due to the large spike from the powerful el Nino of that year). Large fluctuations also make it more difficult to establish the statistical significance of trend, leading to meaningless statements about “no statistically significant warming for 15 years” (or 10 years, or 7, or since last Thursday).
After the natural influences are removed, the adjusted data look like this:
With the bulk of the fluctuations removed, the continued course of warming over the entire time span (including the last decade) is undeniable. It’s worth noting that in all five adjusted data sets, the last two years (2009 and 2010) are the two hottest.
With much of the natural fluctuation removed, it’s possible to compute trends more precisely. Hence it’s interesting to consider whether the trend due to global warming has changed during this interval. To that end, trend rates were estimated (along with uncertainties) for a variety of time intervals, starting with all years from 1979 through 2005 and ending with 2010 (error bars are plus-or-minus 2 standard errors):
None of the data sets shows any evidence that the global warming rate has changed recently. A truly fascinating result is that increased precision enables us to establish the statistical significance of a warming trend using a shorter time span than with unadjusted data. All five data sets show statistically significant warming since 2000.
Another interesting result is that el Nino and volcanic aerosols have a stronger influence on lower-atmosphere temperature (from satellite measurements) than on surface temperature (the plot shows the negative of the coefficient for aerosols, so that for all three factors higher values indicate stronger influence, with black dots for global temperature, red for the northern hemisphere, and blue for the southern hemisphere):
That’s one of the reasons that the satellite data show more natural variation than surface data, as well as greater uncertainty in trend estimates when the known factors are not removed. After removing the influence of known factors, uncertainty levels in trend estimates using surface and satellite data are comparable (again, black dots are for the globe, red for the northern hemisphere, blue for the southern hemisphere):
We can even average the five adjusted data sets, giving this:
That shows, with great clarity and impact, the real global warming signal.
And, it should put an end to real skeptics claiming that global warming has recently stopped or slowed down, because real skeptics base their beliefs on evidence. I don’t expect it will have much effect on the behavior of fake skeptics.
JR: I think it’s also worth noting that whatever slight slowing in global warming some groups may have observed in the past decade, not only was it driven by this “noise,” it was primarily in the surface temperature data set. The oceans kept heating up:
A 2009 NOAA-led study, “An observationally based energy balance for the Earth since 1950” concluded:
[S]ince 1950, the planet released about 20 percent of the warming influence of heat-trapping greenhouse gases to outer space as infrared energy. Volcanic emissions lingering in the stratosphere offset about 20 percent of the heating by bouncing solar radiation back to space before it reached the surface. Cooling from the lower-atmosphere aerosols produced by humans balanced 50 percent of the heating. Only the remaining 10 percent of greenhouse-gas warming actually went into heating the Earth, and almost all of it went into the ocean.
“Total Earth Heat Content [anomaly] from 1950 (Murphy et al. 2009). Ocean data taken from Domingues et al 2008.”
So the warming continues just where scientists expected it.