The biggest question: how much will the world warm?

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.

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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.)


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.


Toxic fear-mongering

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.

Other posts in this series

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 …

  1. Focusing on worst-case climate futures doesn’t work. It shouldn’t work.
  2. The IPCC gives us good news about climate change, but we don’t listen.
  3. Roger Pielke Jr.: the politics of unlikely climate scenarios.
  4. Is climate change an existential threat to humanity?
  5. Another climate scientist speaks out against the hysteria.

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).

The Rightful Place of Science: Disasters and Climate Change
Available at Amazon.


17 thoughts on “The biggest question: how much will the world warm?”

  1. As Curry shows, an honest “business as usual” projection is about 1.0 C additional warming this century. The same result can be obtained just by extrapolating the warming since the mid-20th century; that amounts to assuming that all warming in that period (roughly one full cycle of the main ocean oscillation) is anthropogenic and that CO2 will continue to increase exponentially, producing a linear increase in forcing and temperature.

    The actual increase will almost surely be less. One reason is that technological advances will result in more efficient use of energy and/or new sources, such as safe, affordable nuclear. Another is that carbon emissions level off as economies mature. A third is that some of the recent warming might be recovery from the little ice age.

    Gebbie, using Pages2K data, shows that warming since the mid-19th century has taken place in 3 roughly equal steps (, Figure 1). The first was surely natural, the second was probably at least partially natural, if the third was also partly natural, then the estimate of 1 C further anthropogenic warming is too high.

    Anthropognic warming is something we should keep a wary eye on. It is not a cause for panic.

  2. Dear Larry,

    On this Christmas Eve Day I want to express my gratitude. I am grateful for you, your guest posters and your regular commenters.

    Merry Christmas!
    Happy Chanukah! That’s for Martin, heh heh.

  3. I am left wondering what 1oC means for e.g. ocean acidification. also 1-2ft in sea level is not negligible. will miami still be there? nyc? how much higher will storm surges go and which coastal communities can we expect to be obliterated?

    to say that the extreme alarmists exaggerate is not to say that nothing will change. it is important for more moderate analysts to address the consequences of more moderate change.

    1. jk47,

      (1) “i am left wondering what 1oC means for e.g. ocean acidification”

      Nobody knows with much confidence. The most recent IPCC Report (SR1.5) says this (Chapter 3):

      “Risks of impacts and decreasing food security become greater as warming and acidification increase, with substantial losses likely for coastal livelihoods and industries (e.g. fisheries, aquaculture) as temperatures increase beyond 1.5°C (medium to high confidence).”

      While its nice to know the direction of risk changes (“greater”), policy-makers need to know magnitudes. How do the changes increase with temperature? That is, at 1°C, 1.5°C, 2°C? This is where the scattershot nature of climate research has hurt. Key issues for policy-makers are not the focus of even government-funded research, and little attention is given to replication of findings (necessary to show the robustness of findings).

      “1-2ft in sea level is not negligible. ”

      It is when taking place over 80 years. The seas have been rising since the end of the ice age (in fits and starts). People have adapted and can easily continue to do so.

    2. New York City is still going to be a major city if sea levels rise 2 feet, although I would not be shocked if it makes the southern part of Manhattan less valuable or leads to a major sea wall project. (The big problem would be storm surges, etc. rather than the city becoming Venice during ordinary weather.)

      1. It’s hard to see the funding of what would be huge infrastructure projects-not just a sea wall, but doing something about subway tunnels, power stations and so on. I still remember seeing pictures of Manhattan at night a few days after Sandy. Fully lit down to 14th Street, with the southern part of the island blacked out- what Jon Stewart called”little North Korea.”

        Connects to the problem that this country is basically running on infrastructure built in the 50s and 60s. W we haven’t been willing to spend the money to keep the bridges from falling down

      2. jk47,

        “It’s hard to see the funding of what would be huge infrastructure projects”

        It should not be hard to see. Just look at our past. What we did once, we can do again.

        “Connects to the problem that this country is basically running on infrastructure built in the 50s and 60s”

        That’s a symptom of a very deep problem.

      3. Yes, my hope is the coming crisis will ultimately unite the country around things like renewing and modernizing our national infrastructure. That’s a hope. I wish it were a prediction

  4. The sun is currently in the longest spotless spell since 1913. Maybe the question should be how much will the earth cool?

    1. Jim,

      As Dr. Curry says, the current consensus is that the warming from GHG is a much larger factor than any likely cooling effect from the sun. Also, the GHG warming is a far more established dynamic than the so-far hypothetical solar effect.

      1. Michael S Lorrey

        Problems with these GHG claims is that:
        a) Their CO2 radiative models ignore curvature of the Earth and assume Earth is a flat disk (yes that means alarmists are flat earthers!). This results in an error in the radiative model of between 3-6% over the whole range of troposphere altitudes, with a wider range at the equator than the poles.
        b) GHG climate models ignore atmospheric convection, for the simple reason that calculating the Navier Stokes Equations necessary for accurate modeling would require computers so powerful they’d become the largest carbon footprints on the planet,they’d burn so much electricity. Ask Dr Curry how much of a real model should involve convection?
        c) Solar forcings in their models ignore how galactic cosmic rays, which are modulated by solar magnetic strength, are a major component of cloud formation, and therefore planetary albedo.
        I could go on and on with all the many and sundry biases and flaws in the climate models, without even touching on all the data fraud committed by alarmists doctoring the surface station data with “corrections” that always seem to cause recent temps to warm and older temps to cool, amplifying the alleged warming, and how these same data doctors always seem to delete the original records so that nobody can go back and either check their work, or recover the REAL truth.

  5. I see that this site is one for GW “believers”. I had better move elsewhere quickly. :-)

    The dominant influence on the earth’s temperature is the sun – by far. We currently have an “inactive” sun. That leads to more cosmic rays hitting the earth which in turn leads to more clouds and cooling. Already the growing seasons in the Northern Hemisphere are shortening. Do expect some failed crops and the consequent rise in food prices. Cooling is a far greater threat than warming.

    OK. I am off now. Sorry for spoiling your mood. BTW, temperature data is being manipulated to reduce the highest temperatures of the past.

    1. ALfred,

      “I see that this site is one for GW “believers”.”

      If you believe science is a matter of “believers”, then you “had better move elsewhere quickly.” Better yet, read a Climate 101 textbook.

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