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Human activity continues to warm the planet over the past 16 years

What the science says...

Select a level... Intermediate Advanced

Short term surface temperature trends are strongly influenced by natural and other factors which may mask the slow but inexorable anthropogenic warming trend. However the total energy of the climate system continues to increase.

Climate Myth...

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)

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.

Can we isolate the human contribution to 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. They can be divided into 3 categories: observational biases, natural influences and human influences.

Observational biases

  1. Coverage bias. The HadCRUT4 and NOAA temperature records don’t cover the whole planet. Omitting the Arctic in particular produces a cool bias in recent temperatures. (e.g. Hansen et al 2006, Folland et al 2013). The video avoided this problem by using GISTEMP. However the issue affects the Foster and Rahmstorf analysis of the other records.

  2. Sea surface temperature bias. The GISTEMP and NOAA temperature records don’t include corrections for the transition from warm-biased engine room measurements to buoy measurements over the past 15 years. This produces a cool bias in recent temperature trends, although this result is based on only one study (Kennedy et al 2012).

Natural influences

  1. Volcanic eruptions. The recovery from the Pinatubo eruption is responsible for a short term warming. GCMs predict that this is a significant contribution to recent warming. However Neely et al (2013) find evidence of a significant cooling contribution from recent volcanoes.

  2. The El Nino oscillation. The recent run of La Niñas produces a moderate cooling effect.

  3. The solar cycle. The current low solar activity produces a small cooling effect.

  4. Longer term oscillations, including the AMO and PDO.

  5. Changes in ocean heat uptake. A number of recent papers have found evidence that heat has been going into the oceans rather than the atmosphere recently, see in particular Balmaseda et al (2013), Guemas et al (2013), Nuccitelli et al (2012) and Levitus et al (2012) (and also used in Otto et al, 2013).

Human influences

An increase in other industrial emissions. Unlike long-lived greenhouse gasses such as CO2, short-lived atmospheric constituents can cause significant short term fluctuations in the rate of warming. Chinese aerosol emissions have varied significantly over the past 16 years (Klimont et al 2013). Murphy (2013) finds that the direct cooling effect of these emissions has been limited, however secondary effects on clouds are still uncertain.

Implications

In order to reliably detect a change in the underlying rate of warming, it is necessary to separate out all of these contributions. While attempts have been made at this calculation, (for example Lean and Rind 2008 and Foster and Rahmstorf 2011), there remain significant uncertainties, for example in the duration of the volcanic response and the contribution of recent volcanoes.

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).

where is warming going

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).

Fig 1Figure 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

Last updated on 28 May 2013 by Kevin C. View Archives

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Comments

Comments 1 to 13:

  1. A colleague points out that natural climate variability on decadal (and longer) timescales hasn't been removed. After removing some fast natural variability, the video describes the remainder as "the human contribution to climate change, plus some wiggles due to weather."

    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.
  2. I can give a partial answer to that. On longer timescales you can't approximate the human contribution as linear, so you need to use a method which takes into account radiative forcing. The simplest approach is to use the 1- or 2-box model method of Rypdal to find the response function which maps radiative forcing onto temperature, plus the El Nino term. If you do that (using the GISS forcings and GISTEMP) on 1880-2010 then you get this:



    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.
  3. Another factor that would be nice to mention is aerosols from human activities. I believe those would have contributed to warmning during the 1980's and 1990's, and to a slight cooling during the 2000's. With the effect of those aerosols subtracted, there might even have been a slight acceleration in warming the last decade.
  4. 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

  5. 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.)

  6. Another factor, or maybe a subset of the ocean heat uptake, is the meting of the Arctic Ice. It is noteworthy that the ice melt has speeddecades once 2001. I have calculated that some 10^21 Joules have gone into melting the ice since 1997. http://greenerblog.blogspot.co.uk/2013/07/how-much-heat-has-gone-into-melting.html

  7. Richard, your estimate looks good.

     

    Skeptical Science have used 0.5 W/m^2 as the energy imbalance for Earth... this is the energy going into heating the planet. At 5.1e14 m^2 for Earth's surface, and 3.15e7 sec per year. and for a 16 year period of time, you have about 1.3e23 J of energy heating the Earth. Using 0.8% of this into Arctic Sea ice (from the diagram above) this corresponds to 10^21 J. Same as you have calculated.

  8. Richard, your estimate looks good.

     

    Skeptical Science have used 0.5 W/m^2 as the energy imbalance for Earth... this is the energy going into heating the planet. At 5.1e14 m^2 for Earth's surface, and 3.15e7 sec per year. and for a 16 year period of time, you have about 1.3e23 J of energy heating the Earth. Using 0.8% of this into Arctic Sea ice (from the diagram above) this corresponds to 10^21 J. Same as you have calculated.

  9. Thanks Sylas.  So is it possible and/or justifiable to calculate the surface heating that would have taken place had that 10^21 J gone into warming the atmosphere instead of into melting Arctic ice?

  10. Richard... yes, sort of... though it doesn't mean much. The Earth is heating up, and most of the absorbed energy goes into heating the ocean. According to the figures; 93.4% of it. Of the rest, 2.1% is taken up as heating of continents, and 0.8% is taken up as melting Arctic sea ice. So you can divy up those numbers in various ways. I don't recommend it; it muddles much more than it reveals.

     

    If the sea ice wasn't there to be melted, then everything changes; because it's a complex interacting system we are considering. For example, the loss of sea ice in summer is a significant feedback that contributes to the magnitude of all changes. You could consider a conterfactual in which melting ice isn't particularly endothermic, so that no energy was taken up, any excess energy would be taken up mostly in the ocean; meaning very little difference in land temperatures. We already know that when there's excess energy around, it goes mostly into the ocean.

     

    The other issue is that temperature is not heat.

     

    The temperature we get to is not determined by heat capcities; but simply by what temperatures will bring radiation emitted into balance with radiation absorbed. Heat capacities -- and the absorbing of excess energy -- is part of the process of getting back into balance.. and this is about how long it will take for temperature to stablize for a given atmosphere or forcing.

     

    The energy figures are not really about temperature, but about the imbalance and the time it will take to get to balance again.... whatever temperature that happens to be. Getting rid of the capacity of melting ice to absorb heat would mean only we get to equilibrium temperature a tiny little bit faster. It makes no difference to the temperature we actually reach.

    Response:

    [JH] I deleted your duplicate post of the above. 

  11. But it may affect the *rate* at which we increase global surface temperatures.

    It is clear that energy is taken up by melting Arctic ice. I am not quite clear as to whether this is already accounted for in calculations of ocean heat content, or is it additional to OHC?

    If it is additional, what proportion of the forcing has gone into melting Arctic ice? Could it be a significant co-factor in the slowed rate of increase in land surface temperatures over the last decade?

    There now follows my attempt to answer this question. It comes with health warnings, as I am not a physicist, and am not even very confident with exponentials, so my conclusions may be way out.

    Over 10 years 2002-2912, 10 e21 Joules have been absorbed into the Arctic ice melt.

    So each year, 10e20 Joules were absorbed.

    Since there are 3.15 x 10 e7 seconds in a year, that is equivalent to 3 x 10 e13 Joules per second, in other words, 3 x 10 e13 watts go to melt the ice.

    The earth's surface is 5.1 e 14 square metres. Therefore the quantity of watts per square metre relating to Arctic ice melt is about 6 e-2, or 0.06 w/m2

    The current level of radiative forcing due to GHGs, according to the IPCC AR4, is 1.6 watts per square meter (with a range of uncertainty from 0.6 to 2.4).

    Therefore the effect of the Arctic ice melt is to reduce the effectiveness of the radiative forcing due to enhanced greenhouse gases by 3.75% (range 2.5 - 10%).

    If this calculation is correct, it would seem therefore that the Arctic ice melt, if it is indeed not already accounted for in the OHC figure, is a significant component of the reduction in the rate of surface warming, and very significant al lower estimates of GHG forcing.

    If all planetary ice losses (from glaciers, Greenland, and the Antarctic) were included, the contribution would be even more significant.

  12. Richard Lawson @11, by my calculation you misplaced a decimal point when converting from Joules per annum to Joules per second (Watts).  The correct value for the full caculation is 0.006 W/m^2 of energy used in ice melt given your initial values.  That is approximatly 1% of the TOA energy imbalance.

  13. Thanks Tom. And, having now read Church et al 2011, I see that they included all ice  melting energy in their calculations, so it seems I was on a wild goose chase. Apologies.

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