What can climate modellers tell us about flying and eating beef?

Representative consumption pathways

Representative concentration pathways (RCPs) are hypothetical emissions of greenhouse gasses and other climate pollutants.  (So why are they called concentration pathways?) The RCPs specify individual climate pollutants, such as CO2, CH4, N2O and black carbon for each year from 2000 until 2100. The RCPs were introduced in IPCC Assessment Report Five (AR5) in 2014. After a selection process four of these pathways – tables of numbers specifying the yearly emissions of each pollutant were chosen as representatives of possible future climate forcing over the century.

Four RCP’s were chosen as standard: RCP2.6, RCP4.5, RCP 6 and RCP8.5.  RCP2.6 specifies the lowest concentrations of climate pollutants was specified in RCP2.6.  According to climate models, RCP2.6 is the only RCP that keeps the rise in global average temperature since pre-industrial to below 2°C.  The others have worse outcomes i.e. higher average global temperatures.

Different climate pollutants have different warming and cooling effects on the Earth but the effects of different pollutants are often combined into a figure that would equal the effect of carbon dioxide alone. This measure is called carbon dioxide equivalent or CO2e. Combining the effects of the pollutants for the RCPs give this graph

According to Moss et al, The next generation of scenarios for climate change research and assessment, the global temperature changes in the years to 2100 are given by

Representative Concentration Pathway Temp anomaly °C
  RCP 2.6 1.5 – peak then decline
  RCP 4.5 2.4 – stabilisation without overshoot
  RCP 6 3.0 – stabilisation without overshoot
  RCP 8.4 4.9 – still rising by 2100

Figures taken from Skeptical Science

The United Nations Framework for Climate Change, describe the Paris  Agreement of 2016 thus:

“The Paris Agreement’s central aim is to [ keep] a global temperature rise this century well below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5 degrees Celsius.”

Since the Paris agreement there have been discussions as to a what temperature rise “above pre-industrial levels” means: e.g. What is the baseline?  In the paper, Interpreting the Paris Agreement’s 1.5C temperature limit, Joeri Rogelj discusses natural variability

“Therefore, we argue that the long-term temperature goal in the Paris Agreement should be understood as long-term changes in climatological averages attributed to human activity – excluding natural variability.”

He doesn’t discuss here the issue that has arisen by a paper he co-authored, Emission budgets and pathways consistent with limiting warming to 1.5 °C. This paper has caused some controversy partly because it uses the HADCRUT measure of the temperature.  This rates the global average temperature lower than other measures of temperature because it does not include temperatures in the Arctic, where temperatures are increasing rapidly. The quantity of greenhouse gasses that can be emitted before a particular temperature is reached is the remaining carbon budget, usually expressed as carbon dioxide equivalent.

Criticism of this paper has come from one of the co-founders of Real Climate, Gavin Schmidt. Dr Schmidt is director of the NASA Goddard Institute for Space Studies. On his Twitter feed he said:

“Headline claim from carbon budget paper that warming is 0.9ºC from pre-I is unsupported. Using globally complete estimates ~1.2ºC (in 2015)”

Dr Richard Millar, is the lead author of the “Emissions budgets” paper is a post-doctoral research fellow at the Oxford Martin Net Zero Carbon Investment Initiative where Professor Myles Allen is co-director.  Professor Allen is also one of the co-authors and one of the best known climate scientists in the UK.  It may not be an inaccurate characterisation to call this a debate between Myles Allen and Gavin Schmidt.

Using the HADCRUT as the measure of global average temperature, the”Emissions budgets” paper reports  a smaller rise in temperature since pre-industrial. This helps the conclusion that the world can emit more greenhouse gasses (has a larger carbon budget) before the 1.5°C limit is reached: More greenhouse gasses than other scientists have calculated using  “globally complete” measures of average global temperature, such as GISTEMP from the NASA Goddard Institute for Space Sciences.

Perhaps we should note, whatever the difference between the HADCRUT and GISTEMP measures on this day in October 2017, the actual physical state of the Earth (no) is unchanged. It is what it is, recent floods, droughts, hurricanes, wildfires and all.

The calculations  for remaining carbon budgets are made using climate models.

Future physical states of the Earth are predicted by climate models. These are used to estimate remaining carbon budgets by counting emissions as global temperature changes. However, the same “global temperature” may describe many different physical states. For example global sea level may be different so might the  size of remaining ice masses. Even these two are composite measures that can have different detailed structure. E.g. Will the Himalayan glaciers have disappeared or even more been shaved off the Greenland Ice Sheet?

To gather together all the characteristics that are used in complex climate models into one number, global average temperature, may be a useful shorthand but it is a fudge. It ignores not only regional temperature variations but also other measures, which are also fudges; but like sea levels, remaining ice mass and ocean heat content, they add more to the climate picture necessary for policy making.  And, let’s not forget the problem that looms in many scientists minds: ocean acidification.

Perhaps the “global average temperature” fudge is good for getting political agreements in the hands of skilled political operators but it isn’t enough to drive grown-up policy.  A more detailed picture of future possible climates and their consequences is necessary.

As well more detailed consequences of a changed climate ,  policy makers should know more about the levers of mitigation that are possible. Here, concepts like carbon dioxide equivalent (CO2e) which conflate several different climate pollutants become concepts that can be confusion: e.g. whether the short lived pollutantsin the CO2e composite should be calibrated for 20, 100 or 500 years.

In Well below 2°C: Mitigation strategies for avoiding dangerous to catastrophic climate changes, Ramanathan et. al discuss three possible groups of levers for mitigating climate change. These are (1)cutting CO2 emissions, (2)cutting short lived climate pollutants and (3)sequestering carbon. Although these are levers of a sort, more direct mitigations might be based on altering consumption:

  1. Cut down on aviation
  2. Cut down on fossil fueled land and sea transport
  3. De-carbonise electricity
  4. Build with wood not bricks and steel
  5. Cut beef and lamb consumption
  6. Stop open wood fires

And lots more.

Changing any of these climate levers independently changes the mix of climate pollutants: a change that cannot be represented by Representative Concentration Pathway because the concentrations come from fixed tables of pollutant concentrations.

It would help if the effects of  these climate levers were outputs from climate models – separate effects like sea level rise or Himalayan glacier retreat but driven by measures of consumption like changes in aviation and diet.

Representative Concentration Pathways  used in climate models confuse the effects of consumption-based levers. Will there be, any time soon, climate computer models that can have Representative Consumption Pathways which could tell us the effects on climate of halving air travel or changing our diet from beef and lamb to more climate friendly diet?


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