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Global Warming Scenarios:
will temperatures only go up?

Charlie Nelson
24 March 2011

The future rate of global warming in response to rising concentrations of greenhouse gasses such as carbon dioxide and methane is uncertain due to the complexity of the climate system, especially feedback (both positive and negative) mechanisms.

The Intergovernmental Panel on Climate Change (IPCC) in its Climate Change 2007: Synthesis Report, says (page 30) that "Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level".  The report does acknowledge that some aspects of the climate appear not to have changed, such as air temperatures around  Antarctica.

The same IPCC report contains estimates of future temperature increases based on a range of scenarios.  The ranges of temperature increase by 2090-2099 relative to 1980-1999 start at between 1.1 and 2.9 degrees Celsius for scenario B1 and go up to between 2.4 degrees and 6.4 degrees Celsius for scenario A1F1 (page 45).  The six scenarios evaluated are based on a range of assumptions concerning demographic, economic, and technological change.  They do not include additional climate policies above current ones, so that the impact of effectively doing nothing can be assessed.

For the next two decades a warming of about 0.2 degrees Celsius per decade is projected for a range of scenarios.  The chart on page 46 shows the trajectory of temperature rise, for each scenario, by year.  Temperature rise is depicted as almost monotonic with only small "wobbles" about the trends.  Temperatures in 2020 are depicted as higher than temperatures in 2000 for all scenarios.  Similarly, temperatures in 2040 are depicted a higher than temperatures in 2020 for all scenarios.

But is there not a scenario in which global temperatures do not increase consistently - in particular is there a plausible scenario in which temperature actually goes down for a period?  The IPCC report seems to rule this out completely.

But if average temperature does actually go down for a period of a decade or more, the credibility of the thousands of IPCC climate scientists would be destroyed in the eyes of many and action to limit greenhouse gasses would be postponed for decades at least.

It seems to me that there is such a scenario and that by not acknowledging it, climate scientists have exposed themselves to a risk of degradation of their reputation.  And the world may be exposed to a dangerous outcome.

Chart 1 shows historical global average temperatures from 1850 to 2009.  It was downloaded from on 24 March 2011.  I have included the linear trend line, which indicates a statistically significant upward trend of 0.45 degrees per century.  The probability that this trend is due to chance is less than one in a million.

Chart 1

It is clear that temperature has not risen smoothly throughout this period.  Temperatures actually declined between the late 1870’s and about 1910 and again between the mid 1940’s and the mid 1970’s.  Could this happen again?

Chart 2 shows the deviations from the trend.

Chart 2


Deviations from the trend are indicative of a cycle with a period of about 65 years.  If such a cycle is included in the trend model and projections made to 2080, the temperature could actually fall until about 2040 and not be clearly warmer than now until about 2070 (Chart 3).  The cycle is statistically significant.

Chart 3

But what could cause such a cycle, if it really exists?  There are many factors which influence global temperature.  Short-term variations are caused by volcanic eruptions, for example, by ash particles reflecting heat.  Longer-term factors include sulphur dioxide emissions from coal-fired power stations, which lead to cooling.  Clean air acts in the 1970’s resulted in a reduction of these emissions.  There are also a range of natural factors which are only partially understood.

One of these natural factors is the Pacific Decadal Oscillation (PDO).  This is an El Nino – like pattern of climate variability with similar spatial characteristics but with a much longer period of variability.  The causes are currently unknown, as are its predictability.  Chart 4 shows a measure of the PDO and the deviations from trend of global temperatures.

Chart 4


There is a statistically significant correlation between the PDO and global temperatures (only a one in ten thousand chance that it is a fluke result).  The PDO is strongly correlated with the temperature decline from the mid-1940’s to the mid’1970’s and the subsequent strong upwards trend.  The PDO now appears to have switched to a negative phase and this could be associated with lower than expected temperatures over the next several decades.

While there can be little doubt that rising concentrations of greenhouse gasses will drive temperatures upwards, the trend may not be as smooth as suggested by the IPCC.  Indeed, I believe that I have demonstrated that there is a plausible scenario that temperatures will not rise for several decades, and may even decrease, before rising strongly again in the second half of the century.

While I would not say this scenario is likely, it is not impossible.  What would be the consequences?  And what should citizens, scientists, and governments do?

First, should this scenario play out, climate scientists would be completely discredited because they have not prepared us for such a scenario and explained why it could happen yet still be consistent with global warming caused by human activity resulting in increased greenhouse gas concentrations in the atmosphere.

Second, and conversely, global warming denialists would be vindicated for a considerable period of time.  Especially those who have been arguing that the globe has cooled since 1998.  This is a simplistic argument but it is powerful in the court of public opinion and, to my knowledge, it has never been convincingly, publicly, rebutted by climate scientists.

Third, the general public will further lose the will to act on greenhouse gas reductions.  This is already happening, according to a variety of consumer polls around the world.

Fourth, governments will be under much more pressure from citizens and from companies in fossil fuel industries to shelve any action to reduce greenhouse gas emissions.

The result is likely to be an unstoppable tragedy, if it is not already that.  No action of consequence has been taken in the 20 years since the Rio Earth Summit and if this scenario plays out, it would be likely that no action would be taken for a further half-century at least.

What should we do to avoid this potential awful outcome?  Several things:

First, climate scientists ought to publicly acknowledge the possibility of my scenario and to explain how it would still be consistent with current global warming science.  And that the need for action would be as strong as ever.

Second, concerned people, politicians, and business leaders must devise strategies to reduce greenhouse gas emissions for reasons other than global warming:


·        Reducing energy use through efficiency gains is simply a smart economic decision;

·        Increasing public transport usage to reduce road congestion is also a smart economic decision as it boosts productivity;

·        Investments in alternative sources of energy have benefits such as reducing our dependence on oil, which is harming our economy in the “peak oil” era;

·        Preserving remaining forests, and expanding them, should be undertaken on the basis of protecting diversity and paid for by eco-tourism, concerned governments, and individuals;

·        Encouraging developing countries to reduce population growth is smart insurance against rapidly rising food prices and the prospect of a “peak food” era.

There are many challenges ahead and our progress towards tackling them over the past 20 years has not been good.  This scenario on global warming, perhaps unlikely but still plausible, would put the world into an impossible situation in the next 50 years without some very smart strategies being implemented immediately.