The biggest question: how much will the world warm?

Larry Kummer, Editor

6 years ago

Summary: Let’s pretend for a moment that the climate policy debate is still rational, not what it is – a moral panic. In this pretend world, it would be helpful to have a forecast of the likely warming based on cutting-edge science. Like this great article by climate scientist Judith Curry.

Will the world warm by 3°C?

By Judith Curry.

Is 3°C warming over the 21st century now the ‘best estimate’? Here is a reframing of how we think about climate change over the 21st century, and my arguments that we will see only 1°C of warming.

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 Not as Bad as it Once Looked.” This article is interesting for several reasons, especially since Wallace-Wells has been an ‘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 3°C World is Now ‘Business as Usual‘. The punchline …

“We find that IEA numbers imply that the most likely outcome of current policies is between 2.9-3.4°C warming – which is reduced to around 2.7-3°C 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 3°C could result in warming as little as 1.9°C or as much as 4.4°C.”

The transient climate response to cumulative carbon emissions (TCRE) is defined as the global mean surface temperature change per 1000 GtC (PgC) of anthropogenic CO2 emission. 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 1.5°C (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 {Stated Policies Scenario} and CPS {Current Policies Scenario} 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 forecasts.

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 estimates of this date and how much of the early warming was caused by CO2 emissions. Here are some previous posts about this.

The public looks at the 3°C number and thinks it is 3°C more warming from now, not since the late 19th century. But warming from now is what people care about. When projecting the amount of warming in 2100, why go back to 1900 and {problematically} describe all 20th century warming as ‘manmade’? It is far simpler to bypass the attribution issues of 20th-century warming and begin with an early 21st century baseline period. I suggest using 2000-2014, between the two large El Nino events.

The blame game for 20th-century warming might motivate people to reduce 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 ‘extreme weather’ and call them ‘climate disasters’, few of them have plausible arguments for being substantially associated 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 1°C from the 3°C. Now we are down to 2°C.

The uncertainty about climate sensitivity.

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 the 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.”

There is substantial uncertainty in the observationally-assessed value of TCRE. Similar to the LC18 results {“The Impact of Recent Forcing and Ocean Heat Uptake Data on Estimates of Climate Sensitivity” by Lewis and Curry}, the observationally-based values of climate sensitivity are slightly more than half of the model-derived values.

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

About natural variability.

The IPCC’s 21st century climate change predictions do not include natural variability, they are focused only on manmade climate change. Excerpts from the IPCC AR5 …

“With regard to solar forcing, the 1985–2005 solar cycle is repeated. Neither projections of future deviations from this solar cycle, nor future volcanic radiative forcing and their uncertainties are considered. Any climate projection is subject to sampling uncertainties that arise because of internal variability. [P]rediction of the amplitude or phase of some mode of variability that may be important on long time scales is not addressed.”

Does natural climate variability matter for the climate of the 21st century? Of course it does. The common argument is that natural variability is of small amplitude and we don’t know whether it will contribute to warming or cooling, since we can’t predict it.

Well, is anyone predicting another solar maximum in the 21st century, similar to what we saw in the mid/late 20th century? No. Rather, there are some predictions for solar cooling in the mid 21st century. Whether there will be a major solar minima in the 21st century is highly uncertain, but the more telling point is that no one is predicting a new maximum. In any event, endlessly repeating the 1985-2005 solar cycle doesn’t seem to be a particularly good bet.

Regarding volcanoes, the 20th century was quite benign in terms of volcanic eruptions. There were much worse volcanic eruptions in the 18th and 19th centuries. Is there any particular reason to expect the 21st-century volcanic eruptions to be as benign as the 20th century. You have to go back to the period 1340-1440 to find another century-long period as benign as the 20th-century volcanoes.

Now for the multi-decadal and longer ocean oscillations. For the past 25 years, we have been in a regime dominated by both the warm phases of AMO and PDO. Is anyone predicting that these warm phases will persist through the 21st century? No transitions to the cool phases are expected before mid-century.

While we can’t predict future solar, volcanic and long term ocean oscillation activity, we can expect multidecadal periods in the 21st century where the external forcing tends towards cooling and also the ocean oscillations support cooling, reduced Greenland ice melt, etc.

Net cooling from natural sources of 0.2°C or more is not at all implausible over the 21st century; it is difficult to argue for additional warming from natural sources over the 21st century.

1.2°C minus 0.2°C = 1.0°C

Do we face a dangerous future?

1.0 C warming for the remainder of the 21st century seems pretty benign. But if you add the ~1.0°C warming since 1890, then we are at 2°C – ‘dangerous.’ Two degrees C, and then 1.5C, are the touted values of ‘dangerous’ climate change. I discuss what is ‘dangerous’ and discuss alternative perspectives in these posts.

Simply put, in terms of ‘dangerous’ we are looking at extreme weather events, sea level rise and species extinction. I’ve written numerous posts on all of the above, won’t rehash here, other than to point you to the recent IPCC’s Special Report on Oceans, Cryosphere and Climate, since sea level rise is one issue that is directly and monotonically linked to warming. Their main conclusion regarding sea level rise are as follows.

“Projections of global mean SLR under RCP2.6 result in 0.42 m (0.28–0.57 m; likely range) in 2100.
Projections of global mean SLR under RCP4.5 results in 0.55 m (0.39–0.71 m) in 2100.
Projections of global mean SLR under RCP8.5 results in 0.97 m (0.55–1.40 m) in 2100.”

If you take out the implausible RCP8.5, then we are left with 1-2 feet by 2100, compared to ~7 inch rise in the 20th century. And these values are biased high from climate model simulations that don’t sample the full ‘likely’ range of ECS from the IPCC AR5 – no climate model values between 1.5 and 2.3°C.

The issue of 2°C as ‘dangerous’ is tied to concerns about tipping points, and massive melt of ice sheets that were observed in previous interglacials at comparable temperature. My main response to that concern is a request to paleoclimatologists to sort out what was going in the mid-Holocene ‘climate optimum’, when there is at least anecdotal evidence of much warmer temperatures and higher sea level. (Note re the last 2000 years; I’ve yet see convincing evidence that MBH-style shenanigans have disappeared from PAGES2K, etc.)

Conclusions.

1.2°C of additional manmade warming over the remainder of the 21st century isn’t ‘dangerous.’ Yes, there is substantial uncertainty in how the climate of the 21st century will actually play out, and we will undoubtedly be surprised.

But reframing the ‘warming’ with an early 21st century baseline, rejecting RCP8.5 and using more credible values of TCRE goes a long way towards putting manmade global warming into perspective over the course of the 21st century.

Originally published at Climate Etc. on 23 December 2019.
Reposted under its Creative Commons license.

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Toxic fear-mongering

The Big Heat: Earth on the Brink by journalists Jeffrey St. Clair and Joshua Frank (2018).
The Uninhabitable Earth: Life After Warming by journalist David Wallace-Wells (2019).
Falter: Has the Human Game Begun to Play Itself Out? by journalist Bill McKibben (2019).
Plus endless articles in the media, such as “Planet Earth Is Doomed. How Do I Go On?” by Liza Featherstone at The Nation.

Slowly, more people are beginning to speak out about this misuse of climate science. For example, see “Promoters of Climate Anxiety” by Cliff Mass (professor of Atmospheric Sciences at U Washington), “Why Climate Advocates Need To Stop Hyping Extreme Weather” by Roger Pielke Jr. (U CO-Boulder), and “If You’re Worried About Climate Change, Don’t Misrepresent Climate Science” by Nicholas Grossman (Asst. Prof of Pol Sci, U Illinois at Urbana-Champaign). But too few.

The alarmists dominate the media, and speak more loudly and confidently. So the policy debate is irrational, and likely to remain so until the weather finally resolves it. This might be an expensive solution.

About Judith Curry

Judith Curry retired as a Professor of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. She is now President and co-owner of Climate Forecast Applications Network (CFAN). Prior to joining the faculty at Georgia Tech, she served on the faculties of the University of Colorado, Penn State University and Purdue University.

She has served on the NASA Advisory Council Earth Science Subcommittee, the DOE Biological and Environmental Science Advisory Committee, the National Academies Climate Research Committee, and Space Studies Board, and the NOAA Climate Working Group.

She is a Fellow of the American Meteorological Society, the American Association for the Advancement of Science, and the American Geophysical Union. Her views on climate change are best summarized by her Congressional testimony: Policy Relevant Climate Issues in Context, April 2013.

Follow Dr. Curry on Twitter at @curryja. Learn about her firm, CFAN, at their website.

For More Information

Ideas! For your holiday shopping, see my recommended books and films at Amazon. Also, see a story about our future: “Ultra Violence: Tales from Venus.”

If you liked this post, like us on Facebook and follow us on Twitter. For more information about this vital issue see the keys to understanding climate change. Also, see all posts about uncertainties in climate science, about Judith Curry, and especially these …

Activists don’t want you to read these books

Some unexpected good news about polar bears: The Polar Bear Catastrophe That Never Happened by Susan Crockford (2019).

To learn more about the state of climate change see The Rightful Place of Science: Disasters & Climate Change by Roger Pielke Jr., professor for the Center for Science and Policy Research at U of CO – Boulder (2018).