REVIEW
CLAIM: Climate scientists disagree about how much warming is associated with our emissions and whether this warming is larger than natural climate variability; “Variations in the sun and volcanic eruptions also have a substantial impact [on Earth’s climate], but these are simply unpredictable”.
On 15 April 2024, PragerU posted a short video on their website and YouTube titled “The Good News about Climate Change”, which gathered over 500,000 views combined as of the publication date of this review. The video features a former professor, Dr. Judith Curry, who makes claims about the current state of climate change knowledge – the knowns and unknowns, agreements and disagreements. Below we will share our investigation of some of these claims using scientific evidence, followed by evaluations of these claims from scientists with relevant expertise.
Recent rises in global temperatures are being driven by CO2 emissions from human activity; evidence shows that natural variability cannot account for these changes
In the video, Curry attempts to summarize what climate scientists agree and disagree about on climate change. She claims that they agree on the following: “the average global surface temperature has increased over the last 150 years; humans are adding carbon dioxide to the atmosphere by burning fossil fuels; and, carbon dioxide emissions have a warming effect on the planet.”
The claims above do in fact represent some, but not all, of the findings shared by the Intergovernmental Panel on Climate Change (IPCC), the world’s leading authority on climate science. This is an important distinction because it is misleading to imply that these are the only three things that climate scientists agree on – or, more accurately, that the scientific evidence unequivocally shows. For instance the IPCC also reports that “rising greenhouse gas concentrations are driving profound changes to the Earth system, including global warming, sea level rise, increases in climate and weather extremes, ocean acidification, and ecological shifts.”[1]
It is well established among scientists that humans are adding carbon dioxide (CO2) to the atmosphere by burning fuels, and that CO2 has a warming effect on the planet through the greenhouse effect. While remaining in the atmosphere, CO2 prevents heat from escaping and consequently warms the surface of Earth – a concept that is popularly known as the greenhouse effect. This is a consequence of the properties of CO2, which allow sunlight to pass through to Earth’s surface, but cause CO2 to absorb and re-emit the energy that returns (i.e., infrared radiation emitted from Earth’s surface after absorbing sunlight)[2] . It is also well established that global mean surface temperature (GMST) has been rising for over 150 years, as shown in Figure 1 below. In the most recent year, 2023, the National Oceanic and Atmospheric Association (NOAA) reported that “The average global land and ocean surface temperature for January–December 2023 was 1.18°C (2.12°F) above the 20th century average of 13.9°C (57.0°F)—the highest global land and ocean temperature for January–December in the 1850–2023 record.”
Figure 1 – Observed global mean surface temperatures from 1850 to 2020 using four data sets. The horizontal lines represent different time ranges and are annotated to show the rise in GMST in the respective time periods. Source: IPCC (2021)[1]
Although Curry’s claims above about agreement amongst climate scientists are scientifically supported, that ceased to be the case in her following claim that climate scientists disagree about “how much warming is associated with our emissions” and “whether this warming is larger than natural climate variability”.
“This is absolutely not true”, explained Dr. Ella Gilbert, Regional Climate Modeller at the British Antarctic Survey, “we know unequivocally that human activity is responsible for the vast majority of observed warming, and that natural factors make up a very small proportion of the changes seen in the last few centuries.”
Climate scientists know this because they have investigated the impact of natural inputs (e.g., solar, volcanic, etc.) and human CO2 emissions on global temperatures, and have also compared modern temperature trends to natural variations. This allows scientists to determine the relative contributions of these inputs to global warming and determine how modern global warming compares to natural variability. As shown below in Figure 2, greenhouse gases overall cause the most global warming of all the climate change drivers, and CO2 causes the most global warming of all the greenhouse gases.
Figure 2 – The contributions of different drivers to global warming from the present time period (2010-2019) relative to the time period of 1850-1900. The estimates of warming (red) and cooling (blue) from radiative forcing studies (panel (c)) are based on both direct emissions into the atmosphere and their effect, if any, on other climate drivers. Source: IPCC (2021)[1]
Scientists have also investigated which physical properties control the climate system and have quantified their influence on global temperatures. By incorporating all these physical properties in global climate models, they have been able to simulate the climate from 1850 to present day. As shown in Figure 3 below, the simulation that only included natural variables (solar and volcanic) was unable to match observed global temperature changes over the period of 1850-2020. However, the addition of human drivers – such as CO2 emissions – lead to a much closer match between simulated and observed temperatures. These simulations also show that human greenhouse gas emissions are the only variable that can reproduce observed temperatures; other natural phenomena (i.e., solar and volcanic influences) fail to explain the recent rise in global temperatures.
Figure 3 – Observed and simulated changes in global surface temperature from 1850 to 2020. The black line represents 170 years of observed data and is compared to Coupled Model Intercomparison Project Phase 6 (CMIP6) climate model simulations. The brown area represents the temperature response to both human and natural drivers and the green area to only natural drivers (solar and volcanic activity). Solid colored lines represent the averages, and the shaded areas represent the very likely range for the models. Source: IPCC (2021)[1]
The figures above help explain how human inputs, such as greenhouse gas emissions, have driven recent global temperature changes. However, scientists have also compared these changes to natural variations over longer time periods. After analyzing data from the past 2000 years, studies have shown evidence that modern global warming trends are unusual compared to past variations over this period. For example, PAGES 2K Consortium (2019), explains that over the last 2000 years “the largest warming trends at timescales of 20 years and longer occur during the second half of the twentieth century, highlighting the unusual character of the warming in recent decades.”[3] As shown in Figure 4 below, the black line – representing modern instrumental temperature records – shows that recent temperatures have exceeded the upper range of natural (pre-industrial) warming rates of the last 2000 years.
Figure 4 – 2000 years of global warming/cooling rates averaged across 51 years and based on paleoclimate records. Modern instrumental temperature records shown in black. Data sourced from Neukom et al. (2019) and PAGES 2k Consortium (2019). Source: University of Bern
Note that the warming/cooling rates above are averaged across 51 years; evaluating trends at these timescales (20 years and longer) is important because, as the IPCC explains, “over periods of a couple of decades or less, natural climate variability can dominate the human induced surface warming trend.”[1] That is to say that natural variability has less of an effect at timescales of decades to centuries. So looking at longer time periods allows scientists to better understand how human activities are impacting warming trends, without the ‘noisy’ ups and downs in temperature data observed at shorter timescales. As explained by the IPCC, “over the entire historical period (1850–2019), natural variability is estimated to have caused between -0.23°C and +0.23°C of the observed surface warming of about 1.1°C. This means that either the majority, or all, of the warming has been driven by human activities, particularly emissions of greenhouse gases”[1].
To conclude, Curry’s claim is at odds with the science on this point. Scientific knowledge has demonstrated that the vast majority of the observed warming can only be explained by the forcing created by greenhouse gases added to the atmosphere by human activities. Regarding scientific agreement among climate scientists, Dr. Kerry Emanuel, Professor Emeritus of Atmospheric Science at the Massachusetts Institute of Technology, explained:
“There is a widespread scientific consensus that a strong signal of global mean temperature increase has emerged from natural background variability and that it is not caused by other kinds of radiative forcing changes, such as solar output. Of course, as in all scientific endeavors, there is a minority dissent, but in this case that has been reduced to typical background noise levels.”
Evidence shows that recent climate change is not being driven by solar variability or volcanoes
Over the years, claims continue to pop up regarding the impacts of solar activity and volcanoes on climate change. Science Feedback has addressed many of these in several past reviews, and in each case – now including this film – the claims were inconsistent with available evidence. See examples of these claims in the past reviews linked below:
- Review 1: The sun isn’t responsible for current climate change, contrary to claims in Suspicious0bservers YouTube video
- Review 2: Low solar activity has little effect on Earth’s climate, contrary to claim in The Sun
- Review 3: Claim that current climate change can be explained satisfactorily by natural cycles and volcanic activity does not have scientific support
- Review 4: Evidence greenhouse gasses cause global warming denied by Willie Soon in Tucker Carlson interview, resulting in mass social media climate misinformation
Curry claims that “Variations in the sun and volcanic eruptions also have a substantial impact [on Earth’s climate], but these are simply unpredictable”. Although this matter has been partially addressed above by comparing natural and human climate drivers, we will further investigate below.
To gain expert insight on this matter, we interviewed Dr. Ian Richardson, Principal Research Scientist at the University of Maryland, who studies the interplanetary environment between the Sun and the Earth. After reviewing Curry’s claim, Dr. Richardson commented: “The solar influence on climate appears to be small. Changes in the solar visible and infrared irradiance are only ~0.1%, so the effect is not ‘substantial’ and tracks the 11-year solar cycle, so in this sense the changes are not ‘unpredictable’.”
Dr. Richardson’s comment above aligns with evidence found in past Science Feedback reviews on this subject. For example, one of the Science Feedback reviews linked above explains that the rate and magnitude of modern global warming is too high to be caused by solar variability. As shown in Figure 5 below, solar irradiance and global temperatures show opposing trends; solar irradiance has shown no net increase since 1950, but temperatures have continued to rise. If changes in solar activity were a primary driver of recent climate change, we would expect to see global temperatures decrease with total solar irradiance, but the opposite has occurred in recent times.
Figure 5 – Comparison of the global surface temperature changes (red) and the Sun’s energy that Earth receives (yellow) in watts per square meter since 1880. One can see that since the 1960s, the global temperature and solar activity have varied in opposite directions. Source: NASA/JPL-Caltech
Regarding volcanic influences on Earth’s climate, the IPCC explains that large volcanic eruptions can actually have a cooling effect, as they release small particles into the upper atmosphere which reflect sunlight[1]. However, these effects are short lived; “Volcanic eruptions can cool the climate by a few tenths of a degree for a couple of years, but this is only a short-term effect compared to the long-term warming by human greenhouse gas emissions”,
explained Dr. Georg Feulner, Deputy Head of Research Department at Potsdam Institute For Climate Impact Research. He also commented that Curry’s claim “overemphasizes the role of the sun and volcanic eruptions”. Feulner’s comments can be viewed in more detail near the bottom of this review.
As shown earlier in Figures 2 and 3, other drivers – such as CO2 emissions from human activities – have had a far greater impact on Earth’s climate in recent times. Volcanic eruptions can also increase atmospheric CO2; however, as explained by Tobias Fischer, Volcanologist and Professor at The University of New Mexico, “during a typical year, volcanic eruptions contribute only about 0.006% of the global anthropogenic CO2 flux. The average CO2 emission of a person living in the USA is about 16 tons of CO2 per year[4] . Therefore, in any given year volcanic eruptions produce only about as much CO2 as 130,000 Americans, or less than the population of Wyoming—the state with the lowest population in the US.” More on this topic can be found in this linked review.
Conclusion and Final Remarks
As we have shown above, the claims that Curry makes about climate change drivers are misleading and unsupported. Although Curry accurately listed some (but not all) of the unequivocal climate science findings (e.g., CO2 warms the planet, burning fossil fuels releases CO2, and global temperatures have been rising for 150 years), it was phrased in a misleading way that suggests that those were the only solid findings. This excludes many unequivocal findings that are outlined in the IPCC Sixth Assessment Report, for example. Additionally, her characterization of the uncertainties (i.e., disagreements among climate scientists) does not align with available scientific evidence. The evidence shows that CO2 emissions from human activities are the primary driver of recent rising global temperatures, and there is no evidence that natural variability can account for these changes. On the contrary, natural climate drivers such as solar variability and volcanic activity, have made a minimal impact compared to CO2 emissions from human activities.
The information included thus far has addressed the claims that could be investigated using only scientific evidence, without discussion of policy options – which moves into ‘opinion’ territory. For this reason, we have not discussed the underlying message in the video which suggests that “all things considered, planet Earth is doing fine” and that if we focus only on adapting to effects of climate change, there is reason to be optimistic. However, as explained by Richardson, “choosing between either trying to change climate by moving away from fossil fuels or “adapting” to the effects is a red herring. You can do both.” This is why climate scientists discuss climate hazards in terms of both vulnerability (e.g., lack of proper infrastructure) and exposure (e.g., changing climate conditions). The IPCC explains that “continued GHG [greenhouse gas] emissions will further affect all major climate system components, and many changes will be irreversible on centennial to millennial time scales.”[1] They also explain that the available adaptation options will decrease as global warming increases. This provides further reason to focus on both aspects of risk mitigation – reducing known drivers of climate change and improving infrastructure.
SCIENTISTS’ FEEDBACK
Kerry Emanuel, Professor of Atmospheric Science, MIT:
There is a widespread scientific consensus that a strong signal of global mean temperature increase has emerged from natural background variability and that it is not caused by other kinds of radiative forcing changes, such as solar output. Of course, as in all scientific endeavors, there is a minority dissent, but in this case that has been reduced to typical background noise levels.
On the more consequential question of whether this has caused increases in certain kinds of natural hazards, such as wildfires and hurricanes, there is no general answer that covers them all and we have to look at each hazard in each region.
Since the mid 1990s, North Atlantic tropical cyclone activity, including landfalling storms, have been at levels not seen in records going back to 1900, even though there was some elevation of activity in the 1930s as well as the 1950s and 60s. Only about one third of North Atlantic tropical cyclones affect the continental USA, so we have a problem seeing trends with such small numbers, but the decade of the 2000s far surpassed the 1930s in US landfalling hurricane power [based on IBTrACS data][5].
Ian Richardson, Research Scientist, NASA/University of Maryland:
[In the video, ] there’s a lot of “cherry-picking” of what are generally facts in themselves that are strung together to try to make the point that we shouldn’t move away from fossil fuels.
e.g., the Lake Chad example appears to be an inexpert politician not taking account for what appears to be the actual cause of the lake failing and says nothing about the reality of climate change.
The 97% “per capita” decrease in the effect of bad weather is a rather meaningless statistic since there has been a huge increase in the population – and hence also in the number of people that are likely to be impacted by such a weather event.
Choosing between either trying to change climate by moving away from fossil fuels or “adapting” to the effects is a red herring. You can do both. And it’s only practical to adapt so far. For instance, you can build sea walls, but not everywhere (and how do you choose where? – that’s an economic/political decision that may not be available to poor low-lying countries) and it’s not feasible to keep adding height as sea levels rise. Similarly for controlling water resources which are finite and subject to the effects of climate change.
Any models used in science are imperfect but that doesn’t mean that they can’t provide insight and guidance and should be dismissed. Specifically, they may not include solar and volcanic effects because they are assessed to be unimportant.
The solar influence on climate appears to be small. Changes in the solar visible and infrared irradiance are only ~0.1%, so the effect is not “substantial” and tracks the 11-year solar cycle, so in this sense the changes are not “unpredictable”. The irradiance has also declined since around 1980 as a result of the ~100 year Gleissberg cycle, whereas global temperature has risen during this time. Solar activity is somewhat “unpredictable” on short timescales[6] (days/weeks) and there is debate about whether for example variations in solar X-rays, ultraviolet radiation and energetic particles can influence the atmosphere and ionosphere on such timescales, but that’s confusing weather with long-term climate variability.
Volcanoes are clearly unpredictable. There’s a useful summary at USGS[US Geological Survey]. The main issues with respect to climate change appear to be emissions of sulfur dioxide, which tends to cause atmospheric cooling, and carbon dioxide, which contributes to heating. However, the CO2 contributed by even large volcanoes is small compared to annual (2010) anthropogenic CO2, which is equivalent to 3500 Mount St. Helen’s or 700 Pinatubo eruptions and that volcanos add less than 1 percent of that produced by human activities[7].
Ella Gilbert, Research Scientist, University of Reading:
1. Curry’s claim: “Climate scientists disagree about the most consequential issues: how much warming is associated with our emissions, and whether this warming is larger than natural climate variability.”
This is absolutely not true. We know unequivocally that human activity is responsible for the vast majority of observed warming, and that natural factors make up a very small proportion of the changes seen in the last few centuries. For example, the IPCC’s most recent report (AR6) contains the following in its working group 1 report summary for policymakers (section A1.3):
“It is likely that well-mixed GHGs contributed a warming of 1.0°C to 2.0°C, other human drivers (principally aerosols) contributed a cooling of 0.0°C to 0.8°C, natural drivers changed global surface temperature by –0.1°C to +0.1°C, and internal variability changed it by –0.2°C to +0.2°C.”[1]
(Fig from IPCC AR6 WG1 SPM)
i.e. that the contribution of natural factors to warming is *at very least* ten times smaller than that of greenhouse gases, and possibly very much smaller. The impact of natural drivers may not even be causing warming at all[1].
(Also from IPCC AR6 WG1 SPM)
2. Curry’s claim: “For the past 50 years, the global climate has been fairly benign. In the US, the worst heat waves, droughts, and hurricane landfalls occurred in the 1930s—much worse than anything we’ve experienced so far in the 21st century.”
Climate change has been shown to increase the severity and intensity of extreme events such as wildfires, floods and heatwaves (IPCC, 2021). These kinds of events threaten ecosystems (for instance causing mass coral bleaching events, as have been reported recently[8] – see, e.g. Sully et al. 2019) and carry extreme risk to peoples’ lives (Vicedo-Cabrera, 2021; Lüthi et al., 2023)[9,10] – e.g. 70,000 people died in Europe during the heatwave of 2003 and more than 60,000 died in 2022’s European heatwave (Ballester et al., 2023)[11]. Extremes also threaten livelihoods, especially those based on agriculture and natural resource use.
That the *impacts* of individual extreme events are now generally lower than in the 1930s is due to the fact that we are more prepared and have better tools to adapt and plan for extremes. Besides, people are less vulnerable in other ways (thanks to e.g. better health, fewer labour-intensive outdoor jobs and greater economic support), which means the death toll and losses associated with e.g. droughts, wildfires, heatwaves are lower.
3. Far from being “unreliable” and based on incorrect “assumptions”, models are actually very good at making predictions – Hausfather et al. (2019) shows how well even the oldest, simplest models have performed compared to observed climate change[12]. We *can* predict the big picture of climate change (and do so very successfully) – it’s the regional and small-scale changes that are less easy to predict.
Georg Feulner, Senior Scientist, Potsdam Institute for Climate Impact Research (PIK):
Curry’s claim: “Climate scientists disagree about the most consequential issues”: how much warming is associated with our emissions, and whether this warming is larger than natural climate variability” –
The first part of this statement is misleading, the second part is just wrong. Concerning the first part, the amount of warming associated with emissions is characterized by the climate sensitivity, i.e. the long-term warming after a doubling of the carbon-dioxide levels above pre-industrial concentrations. The latest IPCC assessment puts the climate sensitivity in a likely range of 2.5 to 4 degrees (high confidence)[1], so while we do not know precisely how much Earth will warm under continuing emissions, we are sure that it will warm. Concerning the second part of the statement, the warming already observed in the instrumental record has left the range of natural climate variability.
Curry’s claim: “Variations in the sun and volcanic eruptions also have a substantial impact [on Earth’s climate], but these are simply unpredictable” –
This statement overemphasizes the role of the sun and volcanic eruptions. Solar variability leads to fluctuations of Earth’s global mean surface temperature of about 0.1 degrees, compared to about 1.3 degrees of warming since the pre-industrial era. Volcanic eruptions can cool the climate by a few tenths of a degree for a couple of years[13], but this is only a short-term effect compared to the long-term warming by human greenhouse gas emissions. And the fact that we cannot predict the future behavior of the sun and volcanoes does not imply that we cannot include their effect in a statistical sense in future projections – in which the temperature change is dominated by human emissions in any case.
Comments on other statements in the video:
Curry’s claim: “inadequate climate models driven by unrealistic assumptions”
Climate models are constantly improved and extensively validated against present-day observations and past climate change. They are based on our best knowledge of the physical, chemical, and biological processes in the Earth system and driven by measured (or reconstructed) input data (e.g. on greenhouse gas concentrations) in the past, and scenarios for future emissions for projections. While the models are not perfect (they would not be models, then), they provide important information about future climate change. Climate scientists typically compare the projections of many models to be able to assess robustness and quantify model uncertainty. On the Lake Chad example – one very specific counterexample of environmental change not caused by climate change does not disprove the multitude of expected negative climate change impacts around the world.
REFERENCES:
- 1 – IPCC (2021). Sixth Assessment Report.
- 2 – Zhong and Haigh (2013) The greenhouse effect and carbon dioxide. Royal Meteorological Society Weather.
- 3 – PAGES 2K Consortium (2019) Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era. Nature Geoscience.
- 4 – Le Quéré et al. (2016) Global Carbon Budget 2016. Earth System Science Data.
- 5 – Knapp et al. (2010) The International Best Track Archive for Climate Stewardship (IBTrACS): Unifying tropical cyclone best track data. Bulletin of the American Meteorological Society.
- 6 – Lean et al. (2020) Solar irradiance variability: Modeling the measurements. Earth and Space Science.
- 7 – Gerlach (2011) Volcanic Versus Anthropogenic Carbon Dioxide. American Geophysical Union EOS.
- 8 – Sully et al. (2019) A global analysis of coral bleaching over the past two decades. Nature.
- 9 – Vicedo-Cabrera et al. (2021) The burden of heat-related mortality attributable to recent human-induced climate change. Nature.
- 10 – Lüthi et al. (2023) Rapid increase in the risk of heat-related mortality. Nature.
- 11 – Ballester et al. (2023) Heat-related mortality in Europe during the summer of 2022. Nature.
- 12 – Hausfather et al (2019) Evaluating the Performance of Past Climate Model Projections. Geophysical Research Letters.
- 13 – Robock (2000). Volcanic eruptions and climate. Reviews of Geophysics.
Note: Scientists comments were lightly edited for clarity (i.e., information was added in brackets for context and minor punctuation changes were made).
