Arguments
Human activity continues to warm the planet over the past 16 years
The skeptic argument...
No warming in 16 years
"...there has been no increase in the global average surface temperature for the past 16 years" (Judith Curry and David Rose)
What the science says...
| Select a level... |
Intermediate
|
Advanced
| |||
|
Once natural influences, in particular the impact of El Niño and La Niña, are removed from the recent termperature record, there is no evidence of a significant change in the human contribution to climate change. |
|||||
Update 21/02/2013: Troy Masters is doing some interesting analysis on the methods employed here and by Foster and Rahmstorf. On the basis of his results and my latest analysis I now think that the uncertainties presented here are significantly underestimated, and that the attribution of short term temperature trends is far from settled. There remains a lot of interesting work to be done on this subject.
Global temperatures are affected by both natural and human factors. The human influence is dominated by a slow but inexorable warming caused by greenhouse gas emissions. Natural factors include strong but short lived changes due to El Niño, La Niña and volcanoes.
In order to reliably measure the human influence on climate it is necessary either to use temperature data covering several decades, or to first remove the effect of the natural influences. Both approaches give the same result - human activity is warming the planet.
A media myth has emerged disputing this fact. The myth is based on the fact that temperatures have risen more slowly over the past 16 years than previously. However this period is too short to eliminate the effect of short term natural influences on temperature, and no other attempt has been made to eliminate their effect. Therefore the conclusion is invalid.
The ‘16years’ video demonstrates how the natural influences can be removed from the temperature record, and that the rate of warming due to human activity shows essentially no change.
In order to address the general audience the language in the video has necessarily been heavily simplified. Additional information is provided in the video description and in the advanced version of this rebuttal.
Implications
- The 16-year temperature trend provides no evidence to suggest that human-caused greenhouse warming has slowed.
- The 16-year temperature trend provides no evidence to suggest that the consensus understanding of human-caused climate change is incorrect.
- The temperature record over the past 35 years is consistent with climate change being driven by human greenhouse gas emissions.
- Given that human greenhouse gas emissions are increasing, and that the natural influences do not show a trend on longer timescales, we must expect increasing global warming in the future.
The results of this analysis are consistent with a statement by WMO Secretary-General Michel Jarraud:
"Naturally occurring climate variability due to phenomena such as El Niño and La Niña impact on temperatures and precipitation on a seasonal to annual scale. But they do not alter the underlying long-term trend of rising temperatures due to climate change as a result of human activities"
The rest of the climate system
Focusing on surface air temperatures also misses more than 90% of the overall warming of the planet (Figure 2).
Figure 2: Components of global warming for the period 1993 to 2003 calculated from IPCC AR4 5.2.2.3.
Nuccitelli et al. (2012) considered the warming of the oceans (both shallow and deep), land, atmosphere, and ice, and showed that global warming has not slowed in recent years (Figure 3).
Figure 3: Land, atmosphere, and ice heating (red), 0-700 meter OHC increase (light blue), 700-2,000 meter OHC increase (dark blue). From Nuccitelli et al. (2012).
References
- Foster and Rahmstorf (2011), Global temperature evolution 1979–2010 doi:10.1088/1748-9326/6/4/044022
- Rypdal (2012) Global temperature response to radiative forcing: Solar cycle versus volcanic eruptions doi:10.1029/2011JD017283
- Nuccitelli et al. (2012) Comment on Ocean heat content and Earth's radiation imbalance. II. Relation to climate shifts doi:10.1016/j.physleta.2012.10.010
Credits: Calculations and video: Kevin C. Voiceover: Daniel Bailey. Advice: The SkS team.
Last updated on 21 February 2013 by Kevin C. View Archives
This is probably the right level of detail for a 2 minute video or a basic article. But the intermediate and advanced articles could mention that the remainder is the human contribution plus weather, plus decadal and longer natural variability.
Are most modes of decadal and longer natural variability internal to the climate system, or do they involve radiative forcings that haven't already been subtracted in the video?
If they're mostly internal, I think this point is already indirectly addressed via your graph of heat content. Internal climate variability should swap heat between the ocean and the surface, but all parts of the climate are warming. This could place a bound on the percentage of the surface warming trend which could be due to natural internal climate variability.
While limited to annual data and finishing at 2010, the model shows the same slowdown post 1998, and for the same reason as in the video - the trend in ENSO. In fact the ENSO term is almost identical (marginally larger) to the value used in the video.
This very simple model (20-30 lines of R or python) gives a very good fit of temperature from forcing and ENSO without invoking any multidecadal oscillations with an R2 or 92%. On the basis of this analysis at least there is no justification for invoking longer term climate cycles.
That would seem to settle the issue, however the case isn't completely closed. The result does depend on the choice of forcings. If instead you use the Potsdam forcings, the ENSO term is the same so the conclusions of the video are unaffected, but there is room for a small contribution from a multidecadal oscillation. I've been looking into the differences in forcings and understand some of the issues, but there are others I need to track down.
One other slight complication - there was a slight reduction in the forcing trend in the early '90s, I believe related to the phaseout of CFCs. That should also produce a slight change in temperature trend. But it's probably too small to detect over a 20 year period.
As I understand it, aerosols include particulate matter. Over the past couple of weeks we have seen news about air pollution in China as they close down factories and limit automobiles in the capital. Today, Japan is complaining about the air pollution coming over from China. How much of the aerosol load which is wafted up into the atmosphere is from this source and do we have any information on whether the load of aerosols in the upper atmosphere has been increasing along with China's increased manufacturing. I have heard an estimate that if we stoped the production of all aerosols, we might have as much as a 20C rise in temperature. A sobering thought if China (and the rest of us) cleaned up our act. Was the temporary flattening of the temperature record following the 40's due to American air pollution which they then cleaned up,
http://www.aip.org/history/climate/aerosol.htm
Yes, it seems probably that the aerosol cooling effect has been increasing. Unfortunately the effect is geographically dependent and not well measured.
The point of the video is that at this point I don't think we can detect that effect in the instrumental temperature record with any confidence. (There's an update coming which will show a small change, but still in the noise range.)