Human activity continues to warm the planet over the past 16 years
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.
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:
|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 126.96.36.199.
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).
- 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