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...
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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. |
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Update 26/05/2013: The '16 years' video, originally linked from this article, is not representative of the scientific consensus. In fact the short term trends are rather more complicated. The problem is explained in more detail in this article.
Humans have continued to contribute to the greenhouse warming of the planet over the past 16 years. The myth arises from two misconceptions. Firstly, it ignores the fact that short term temperature trends are strongly influenced by a variety of natural factors and observational limitations which must be analyzed to isolate the human contribution. Secondly it focuses on one small part of the climate system (the atmosphere) while ignoring the largest part (the oceans). We will address each of these errors in turn.
What factors influence the 16 year trend?
Climate scientists have traditionally looked at climate over long periods - 30 years or more. However the media obsession with short term trends has focussed attention on the past 15-16 years. Short term trends are much more complex because they can be affected by many factors which cancel out over longer periods. In a recent interview James Hansen noted "If you look over a 30-40 year period the expected warming is two-tenths of a degree per decade, but that doesn't mean each decade is going to warm two-tenths of a degree: there is too much natural variability".
The list of factors which can affect short term temperature trends is extensive, and some of them can rival the global warming signal in magnitude over short periods. The following table identifies a range of influences on the recent temperature trend:
| Influence | Effect | Notes |
| Human GHG emissions | Warming | |
| Human sulphate emissions | Cooling | Recent emissions from China |
| Coverage bias | Cooling | HadCRUT4 and NOAA only |
| Sea surface temperature bias | Cooling | GISTEMP and NOAA only |
| The El Niño oscillation | Cooling | The recent run of La Niñas |
| Volcanic erruptions | Warming | Recovery from Pinatubo erruption |
| Solar cycle | Cooling | Recent solar minimum |
| Longer term oscillations | Unknown | AMO and PDO |
| Change in ocean heat uptake | Cooling | Balmaseda et al (2013), Guemas et al (2013) |
Most of the short term influences, with the exception of greenhouse gas emissions and probably volcanoes (but see Neely et al 2013), have had a cooling influence. As a result it is unsurprising that we have seen a reduced rate of warming over the past 16 years. The fact that there has been any warming at all is strongly supportive of the warming effect of greenhouse gas emissions.
The fundamental mechanism of global warming is a change in the top-of-atmosphere energy balance, and as a result the energy content of the climate system provides a more direct measure of global warming which avoids many of these problems, although the observational record is shorter and less complete (e.g. Church et al 2011).
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
- 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 28 May 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.)