Judith Curry: 3°C or 1°C?

  • Date: 26/12/19
  • Judith Curry, Climate Etc.

Is 3 C warming over the 21st century now the ‘best estimate’?  A reframing of how we think about climate change over the 21st century, and my arguments for 1 C.

There has been much discussion over on twitter of the new article by David Wallace-Wells:  We’re Getting a Clearer Picture of the Climate Future — and It’s n Not as Bad as it Once Looked.  ‘This article is interesting for several reasons, especially since Wallace-Wells has been ‘alarmist in chief.’

Simply put, it is now becoming more widely accepted that RCP8.5 concentration/emissions scenario is highly implausible.  See my previous post:

Is RCP8.5 an impossible scenario?

A new article by Zeke Hausfather and Justin Ritchie at the Breakthrough Institute is entitled ‘A 3C World is Now ‘Business as Usual‘.  Punchline:

“We find that IEA numbers imply that the most likely outcome of current policies is between 2.9-3.4C warming — which is reduced to around 2.7-3C warming if countries meet their current Paris Agreement commitments.

Uncertainties surround this projection, of course. For one, there are uncertainties in the sensitivity of the climate to rising atmospheric greenhouse gas concentrations that mean emissions expected to produce warming of around 3C could result in warming as little as 1.9C or as much as 4.4C.”

They calculate the amount of warming based on TCRE:

The amount of warming the world is projected to experience can be pretty closely approximated solely based on cumulative CO2 emissions. This relationship between temperatures and cumulative emissions is referred to as the transient climate response to cumulative carbon emissions, or TCRE. Using the TCRE values developed in the IPCC Special Report on 1.5C (SR15), we can calculate the amount of warming expected over the remainder of the century in our extended IEA scenarios, as well as the uncertainty introduced by the wide range of possible climate sensitivity values.”

In the figure below we show the amount of warming between the last decade of the 20th century and the temperature of the late 1800s (which is somewhat representative of preindustrial temperatures) for the four RCP scenarios used in the IPCC AR5 and the extended IEA STPS and CPS cases — assuming flat emissions in each after 2040. The width of each bar reflects the 90th percentile range of warming given the uncertainty in climate sensitivity, while the central point represents the average of all the climate models running that scenario.”

This is a nice analysis by Hausfather and Ritchie.  Some questions, suggestions and criticisms are outlined below:

Baseline

The 3 C estimates in the paper by Hausfather and Ritchie are based on a baseline period  1880-1900.    The canonical rationale is for ‘preindustrial’, which would be mid 18th century, as the Northern Hemisphere was coming out of the Little Ice Age (hardly a climate ‘optimum’).  But then, ‘good’ data is available only since the late 19th century.

The rationale for a baseline for manmade global warming in either the 18th or 19th century is that this is when manmade global warming began. There are multiple takes on this, and how much of the early warming was caused by CO2 emissions.  Here are some previous blog posts:

* Assessing the causes of early industrial era warming

* Early 20th century global warming

* Modern global warming

The public looks at the 3 C number and thinks it is 3 C more warming from NOW, not since the late 19th century.  Warming from NOW is what people care about.

In terms of projecting the amount of warming in 2100, what is the point in going back to 1900, and including all of the 20th century warming as ‘manmade’?  It is far simpler to bypass the attribution issues of 20th century warming, and start with an early 21st century baseline period — I suggest 2000-2014, between the two large El Nino events.

In terms of policy, what matters is how much warming we can expect over the 21st century.  Yes, the blame game in terms of 20th century warming is useful in terms of motivating people to act on reducing fossil fuel emissions.  But at this point, what matters for decision making is how much warming we can expect over the remaining 80 years of the 21st century.

While we complain about the 21st century ‘weather’ and now call them ‘climate disasters’, few of them have plausible arguments for being associated in any way with manmade climate change.  Overall the weather in the early 21st century is relatively benign by the standards of the Little Ice Age or even the early 20th century. The slow creep of sea level rise started circa 1860, well before there was significant manmade global warming.

If you start from an early 21st century baseline, you can subtract 1C from the 3C.  Simple . . .  now we are down to 2C.

TCRE

Nic Lewis wrote a previous post on TCRE: Climate sensitivity to cumulative carbon emissions.   Excerpt:

There are two principal metrics for sensitivity to cumulative carbon emissions. The best known is the transient response to carbon emissions (TCRE). This measures the change in global mean surface temperature (GMST) at the end of a period, typically of the order of a century long, during which CO2 is emitted smoothly. TCRE is stated per 1000 GtC (≡ 1 TtC) emissions, and usually assumes a total of 1000 GtC is emitted. Note that 1000 GtC is the carbon content of 3667 GtCO2.

In CMIP5 earth system models (ESMs), which couple carbon cycle models with atmosphere-ocean global climate models, TCRE ranges from 0.8°C to 2.4°C, with a mean of 1.6°C. The assessment in AR5, which largely mirrors the CMIP5 ESM range, was that the TCRE is likely between 0.8°C to 2.5°C, for cumulative CO2 emissions less than about 2000 GtC, until the time at which temperatures peak. ”

Nic calculated the observationally-based values of TCRE to be 1.05°C.

The observationally-based TCRE estimate of 1.05°C, although within the AR5 range and the almost identical CMIP5 ESMs model range, is little more than half the level reflected in the central RCP scenario projections in the AR5 SPM.10 chart. Assuming that the 1.05°C estimate is realistic going forward, the IPCC’s chart overstates expected 21st century warming by a factor of approaching two, for all scenarios.”

Yes, there is uncertainty in the observationally-assessed value of TCRE.  Similar to the LC18 results, the observationally-based values of climate sensitivity are slightly more than half of the model-derived values.

Lets do math.  With a different baseline, we are now down to 2C.  Multiply 2C by 0.6 (reduced values of TCRE) to yield a warming of 1.2C.

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