IPCC: Guide to Best Breakdowns and Takeaways

August 10, 2021

It’s going to take a while to dig into the latest IPCC report (sometimes called AR6 – for Sixth Assessment report) which was released yesterday.
The report itself is available here.

As always, I’ll be relying on folks that know a whole lot more than I do to help me understand the nuances. Here’s a guide to some of the best breakdowns, analysis, and takeaways. I include here small snippets, but encourage all to follow the links for more details. This is going to take awhile.

Bob Henson at Yale Climate Connections:

In an arresting new look at observed global temperature, IPCC has updated its famous “hockey stick” graphic, so named because of the sharp upward bend since 1850 as compared to past centuries. Featured in the IPCC’s third full assessment report in 2001, the hockey stick became a flashpoint of contention in the world of climate science denial; however, numerous studies have since borne out the concept’s validity.

The long end of the hockey stick now extends back to the year 1 AD, and warming since 2000 has only lengthened the uptick at the end (see below left).

Global temperature has risen more since 1970 than in any half century going back to (and before) the days of Caesar, Cleopatra, and Christ. To arrive at a multicentury period warmer than 1850-2020, one has to go back to before the last ice age, more than 100,000 years ago.

The new report also updates one of the most powerful pieces of evidence for human-produced climate change: a comparison of model portrayals of global temperature since 1850 from two sets of models, one including and one excluding the last 170 years of emissions from fossil fuel burning (see below right). Without these human-produced greenhouse gases, the warming since 1850 simply doesn’t happen.

Gavin Schmidt at RealClimate:

6. Don’t mention the hiatus: Readers will probably remember the prominence of the ‘hiatus’ in the discussions around the AR5 report (written in 2013) (see herehere, or here). Due in part to (IMO) an over-reliance on a single temperature record (HadCRUT4), and (it turns out) non-climatic biases in the ocean temperature records, the trends from 1998-2012 got a specific call out in the AR5 SPM:

In addition to robust multi-decadal warming, global mean surface temperature exhibits substantial decadal and interannual variability (see Figure SPM.1). Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to 0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade)5. {2.4}

Section B.1, SPM AR5

The observed reduction in surface warming trend over the period 1998 to 2012 as compared to the period 1951 to 2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from natural internal variability, which includes a possible redistribution of heat within the ocean (medium confidence). The reduced trend in radiative forcing is primarily due to volcanic eruptions and the timing of the downward phase of the 11-year solar cycle. However, there is low confidence in quantifying the role of changes in radiative forcing in causing the reduced warming trend. There is medium confidence that natural internal decadal variability causes to a substantial degree the difference between observations and the simulations; the latter are not expected to reproduce the timing of natural internal variability. There may also be a contribution from forcing inadequacies and, in some models, an overestimate of the response to increasing greenhouse gas and other anthropogenic forcing (dominated by the effects of aerosols). {9.4, Box 9.2, 10.3, Box 10.2, 11.3} 

Section D.1, SPM, AR5

Now however, the updates to the historical warming, the use of four datasets instead of one, and of course, the series of record breaking years subsequently (2014, 2015, 2016/2020), the issue of variability in decadal trends is no longer so salient. The shifts in the quoted trends (1998-2012 is now 0.12ºC/decade, 1951-2012 is 0.13ºC, HadCRUT5) underlines the trivialness of the issue. To be fair, there is one mention of the hiatus in the AR6 Technical Summary: 

The observed slower global surface temperature increase (relative to preceding and following periods) in the 1998–2012 period, sometimes referred to as ‘the hiatus’, was temporary (very high confidence). The increase in global surface temperature during the 1998–2012 period is also greater in the data sets used in the AR6 assessment than in those available at the time of AR5. Using these updated observational data sets and a like- for-like consistent comparison of simulated and observed global surface temperature, all observed estimates of the 1998–2012 trend lie within the very likely range of CMIP6 trends. Since 2012, global surface temperature has warmed strongly, with the past five years (2016–2020) being the hottest five-year period between 1850 and 2020 (high confidence). {2.3.1, 3.3.1, 3.5.1, Cross-Chapter Box 3.1}

AR6 Cross-Section Box TS.1

Let this episode stand as a clear reminder for assessment reports not to get ahead of the science…

Zeke Hausfather in Carbon Brief:

AR6 uses a combination of historical observations, climate models and an updated estimate of climate sensitivity to provide a best-estimate that the world will pass – or temporarily “reach” – 1.5C somewhere between 2030 and 2035, depending on the future emissions scenario. 

Even in the most stringent mitigation scenario examined in the report – SSP1-1.9 – the world exceeds 1.5C in most models during the middle of the 21st century, before falling back down below 1.5C by 2100 due to the large-scale deployment of negative emissions technologies (for more details on the Shared Socioeconomic Pathways (SSPs) used in the AR6, read Carbon Brief’s explainer). 

The date that the world is expected to pass 1.5C is in “the early part of the range” suggested in Chapter 1 of the IPCC’s 2018 special report on 1.5C (SR15), due to a combination of revisions to historical temperature records and higher near-term warming projections. However, a separate, more directly comparable estimate found in Chapter 2 of the SR15 report is nearly identical to the new AR6 estimates.

The world is expected to pass 2C in emissions scenarios that do not feature strong near-term mitigation with a best-estimate of between the early 2040s and the early 2050s. These – and the 1.5C exceedance dates – are quite similar to those given in an earlier Carbon Brief analysis published in late 2020.

AR6 has also updated the remaining “carbon budget” that can be emitted before the world is committed to 1.5C or 2C of warming. 

The carbon budget for a 50% chance to limit warming to 1.5C is quite similar to that given in the SR15 report. It shows that the world can emit around 460bn tonnes of CO2 (GtCO2) – or just 11.5 years of current (2020) emissions – after 1 January 2021 before being committed to 1.5C. 

The carbon budgets for limiting warming to 1.5C with a 66% chance has been slightly increased due to the narrower range of climate sensitivity in the AR6. The remaining carbon budget to limit warming to 2C is similarly affected.

3 Responses to “IPCC: Guide to Best Breakdowns and Takeaways”

  1. pendantry Says:

    This is going to take awhile.

    Not at all. Summary: ‘AR6’ says much the same as ‘AR1’: we’re fucked unless we do something. Caveat: it’s too late, we’re even more fucked now because nobody’s going to do anything this time, either.

    • ecoquant Says:

      But we know much more precisely how fucked we are!

    • rhymeswithgoalie Says:

      “…nobody’s going to do anything this time, either.

      That’s just it. So many did a lot, and others did what they thought was a lot, but it takes a teeny-tiny percentage of the world’s population (in board rooms and strategy meetings) to undermine it all.

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