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Climate Change: Robust Evidence of Causes and Impacts

Seaver Wang and Zeke Hausfather

In the Fall 2019 issue of the Forum on Physics and Society, Dr Wallace Manheimer penned a lengthy critique of what he saw as flaws in the scientific consensus on anthropogenic climate change. Considering the long history of climate change discourse, earth scientists have by now become acutely familiar with the most common approaches used to question the evidence for global warming, and indeed Manheimer offers little intellectually new.

Menheimer’s primary allegation is that the media regularly overstates the scientific basis for climate change and demonstrates bias by refusing to consider more skeptical viewpoints. Saying that media reporting occasionally overemphasizes aspects of climate change would be fair - exaggerating the Amazon rainforest’s role in providing global oxygen and misinterpreting the IPCC’s Special Report on Global Warming of 1.5C to arrive at a 12-year “deadline” for averting a global mean temperature increase of 1.5C, for instance. Pointing out these and other inaccuracies would represent useful critique and help rectify prevailing misperceptions on certain climate topics. However, Manheimer’s accusation is actually directed at a different target, as in his view the media errs by even attributing climate change to rising greenhouse gas concentrations in the first place. His problem is fundamentally a problem with climate science, and he expends the majority of his efforts in attacking the overwhelming scientific evidence of anthropogenic climate change.

The media, while guilty of some misrepresentations of climate science, has generally represented the scientific literature on climate accurately. The vast body of research supporting ongoing climate change and its dominant anthropogenic drivers represents the sum of impressive scientific efforts spanning disciplines, continents, and generations of investigators. Manheimer’s arguments in the face of such abundant evidence offers us as earth scientists an opportunity to demonstrate the robustness and rigor of the basis for climate change.

Here we respond to the multitude of disparate issues raised by Manheimer in his article, going through them one by one.

Claim 1: Climate change pins all the blame on fossil fuel emissions of carbon dioxide even though the climate is a complex, poorly-understood system dependant on many factors

Response: This view misrepresents climate science as attributing climate change solely to CO2, which is not a claim that the earth science community has ever made. To the contrary, anthropogenic climate change represents the product of multiple human influences in addition to fossil fuel emissions of carbon dioxide, including but not limited to deforestation, release of chlorofluorocarbons, nitrous oxide emissions, tropospheric ozone, methane emissions from ruminant livestock and rice paddies, and reductions in ice and snow reflectivity from emissions of particulate black carbon.

Claim 2: Earth scientists propose the single solution of stopping fossil fuel use, and do not even consider whether the climate effects of ceasing CO2 emissions might be harmful.

Response: The research community neither suggests total cessation of fossil fuel use, nor does reducing fossil fuel consumption represent the sole solution to climate change. Some sectors of the global economy such as steel manufacturing will likely prove very resistant to decarbonization, and so a degree of continued fossil fuel use over the 21st century is in fact virtually guaranteed. Rather, climate change mitigation relies upon achieving net zero emissions or better - a state where greenhouse gases are being added to the atmosphere at a lower rate than they are being removed through natural and/or industrial means. With world population projected to grow to 9-10 billion by mid-century (Connor and Mínguez, 2012), future changes to agriculture will also play a significant part in reducing humanity’s climate impact. Finally, we point out that suggesting that burning fossil fuels produces minimal climate impact while in the next breath sowing fears about the negative climate impacts of ceasing fossil fuel use is quite contradictory.

Claim 3: Glaciers have been receding for 200 years, therefore predating industrial-era emissions of CO2. The Jakobshavn glacier in Greenland that everyone thought was receding is in fact growing, according to a recent study.

Response: Manheimer’s claim that glaciers have been retreating for 200 years is uncited, making it difficult to ascertain where it originates from. Very few glaciers worldwide have been accurately monitored for at least two centuries. Manheimer may be referring to a recent study by Dickens et al., which used oxygen isotope data as a proxy for glacial discharge to identify an increasing trend in Antarctic ice shelf thinning over the past 300 years (Dickens et al., 2019). If anything, however, a longer-term trend of glacier retreat represents a cause for more worry, not less, as such glaciers may have already been predisposed to retreat prior to the onset of human-induced warming, suggesting that climate change could significantly accelerate this pre-existing trend (Dickens et al., 2019). Indeed, the World Glacier Monitoring Service has reported demonstrable recent acceleration in the rate of ice mass loss for the overwhelming majority of monitored glaciers in recent decades (WGMS, 2013).

Manheimer fails to mention that the Jakobshavn glacier study’s authors believe that its recent growth is only temporary and attribute it to natural cyclical variability in regional temperatures (Khazendar et al., 2019). Prior to this shift to recent ice mass accumulation, the Jakobshavn had retreated at an accelerating rate for two decades (Khazendar et al., 2019), single-handedly contributing to an estimated 0.9 mm of global mean sea level rise between 2000 and 2010 (Howat et al., 2011).

Claim 4: Looking at surface temperature data, the increase in global mean temperature has plateaued over the past 20 years.

Response: To the contrary, the reverse is true. Global surface temperatures have continued to increase over the past 20 years, and the rate of warming has in fact accelerated over the period from 1998-2019. The plot Manheimer displays is out-of-date, only including data up to ~2014. The same dataset extended to the current day shows that the warming trend has markedly continued. Interested readers can access the same data themselves.

Figure 2. NOAA global surface temperature anomalies over time, 1880-present, plotted with respect to the 20th century average.

The rate of warming over the past 20 years – the post-1998 period that was highlighted as a warming “hiatus” in the past, is now actually faster than the three decades pre-1998.

Figure 3. NASA global surface temperature anomalies over time, 1970-2019, with 1970-1997 and 1998-2019 trends highlighted.

Claim 5: The 1.5C warming target is less than the temperature difference between New York City and Boston, yet is identified with severe climate change consequences. If we have already warmed by 1C since the pre-industrial era, why will 1.5C cause such severe consequences?

Response: Comparing 1.5C of global mean warming to the temperature difference between New York and Boston is nonsensical. The 1.5C figure represents a global spatial average as well as a temporal average - projected warming will not occur evenly across the entire world, nor will it apply evenly across the annual cycle of temperature. The poles, for instance, are warming at a much faster rate than the rest of the planet, with the Arctic Ocean currently experiencing a rate of warming of 0.5C per decade (IPCC SROCC Ch3). Land areas are expected to warm around 50% faster on average than the global mean. Even a modest degree of additional warming, evaluated over the course of a year, lengthens the warm period of the year and shortens the cold season, altering the seasonality of fires, rainfall, and ice melt, changes that can spark considerable ecosystem shifts.

The current pace of warming will see global mean temperatures well exceed 1.5C warming by 2100, likely in the range of 3-5C warming under no-policy scenarios (Hausfather, 2019), so focusing on the consequences of a 1.5C warmer world alone is fallacious to begin with. The climate impacts of a 3-4C global mean temperature increase would be disproportionately more severe than 1.5C warming and reflect a simultaneously more threatening and more likely future that climate mitigation efforts today are seeking to avoid.

Claim 6: Mainstream media claims that sea levels will rise by 10 feet by 2100, with some commentators claiming a 30-foot increase in this century. During the end of the last ice age, sea level rise took place at a rate of just 1 m per century [1 cm per year], yet you expect me to believe that we could get 3x to 10x that rate of sea level rise because of “a small increase of a trace gas in our atmosphere”?

Response: There is no basis for any claims that sea levels will rise 30 feet by 2100. The IPCC estimates a sea level rise on the order of 0.6-1.1 m by 2100 (2-3.6 ft) for RCP8.5, which arguably represents a worst-case emissions scenario (IPCC SROCC Ch4).

The article referenced as estimating a potential for sea level rise of up to 10 ft by 2100 falls at the very high end of predictions made to date (DeConto and Pollard, 2016) and in particular adopted an aggressive approach to ice cliff instability (Edwards et al., 2019).

Contrary to Manheimer’s assertion, Chris Mooney’s article in the Washington Post does not claim that sea levels will rise by 30 feet by 2100. The scientific paper described in this article rather found past sea levels 125,000 years ago to have been 20-30 feet higher due to large-scale melt of the East Antarctic ice sheet at that time (Wilson et al., 2018).

Regarding rates of sea level rise, it is fallacious to claim that just because sea level rates fell within a certain range during a specific period in the past, that it is impossible for the rate of sea level rise to exceed that range. Indeed, the IPCC predicts a high likelihood of sea level rise exceeding several cm per year by the 22nd century under a high-emissions RCP8.5 scenario (IPCC SROCC Ch4).

Claim 7: The rate of sea level rise has decreased since 1960.

Response: The opposite is in fact true. The rate of global mean sea level rise has significantly accelerated since 1960, climbing to a current day pace of 3.58 mm/yr (IPCC SROCC Ch 4). In comparison, the rate of sea level rise from 1901-1990 was approximately 1.4 mm/yr.

Figure 4. (Top) Global mean sea level over time, relative to the 1992-2006 average. (Bottom) Rate of global mean sea level rise (mm/yr) over time. Chart by Zeke Hausfather/Carbon Brief.

Claim 8: Perhaps by 2100 ice caps will all melt based on models, “as many speculate now”.

Response: This is an erroneous claim. No model projections anticipate full loss of all ice caps in 2100 or anything approaching it. Loss of major ice sheets in response to climate change is a long-term process taking place on a time frame of centuries if not millennia (IPCC SROCC Ch 3; IPCC SROCC Ch 4).

Claim 9: James Hansen’s 1988 climate modeling paper overestimated global temperatures by 150% compared to real observations made in the decades that have followed its publication, with observed temperatures below even their lowest-emissions scenario in which CO2 emissions stopped growing in the year 2000. CMIP5 models have continued to over-predict temperature increases.

Response: An updated analysis with data extended to the present day indicates that contrary to Manheimer’s assertion, the middle-case model scenario utilized by (Hansen et al., 1988) has overall predicted the increase in global mean temperature quite well. Current temperatures are closely in line with the range of scenario predictions from what is now a rather dated model - a remarkable achievement. The figure cited by Manheimer only extends until 2012, the year in which the largest divergence between Hansen et al.’s model and observations occurred. Later generations of climate models with significantly-improved capabilities have continued to demonstrate a strong ability to predict global temperatures that we have observed since (Hausfather, 2017; Hausfather et al., 2019).

(Hansen et al., 1988) employed a climate sensitivity that would be considered on the high end of most models that are utilized today. The 150% overestimation figure Manheimer cites also misleads readers by comparing observed temperatures to the very worst-case emissions pathway modeled by Hansen’s team, which modeled emissions increased exponentially at rates well above those actually observed over the same period.

Figure 5. Projected warming from Hansen et al 1988 (scenario B–thick black line–and scenarios A and C–thin solid and dashed grey lines). Chart by Zeke Hausfather/Carbon Brief.

Here, we show a comparison of modeled (CMIP5) versus observed global mean surface temperatures. This analysis demonstrates an extremely strong agreement between observed and modeled temperatures, with observations falling entirely within the envelope of CMIP5 modeled temperatures (light dotted lines) and closely matching the blended multi-model mean.

Figure 6. Projected warming from the IPCC Fifth Assessment Report (mean projection–thick black line, two-sigma upper and lower bounds shown by thin dotted black lines). Dashed black line shows blended model fields. Chart by Zeke Hausfather/Carbon Brief.

Claim 10: Damaging hurricanes are blamed on climate change, but the frequency of hurricanes has not increased, and may even have decreased since the 1930s!

Response: Manheimer leverages a figure borrowed from a blog showing the number of Category 3+ hurricanes per year that have made landfall on the US mainland from 1851 to 2018. We first note that binning the number of Category 3+ hurricanes making landfall over US soil represents a flawed metric with which to assess overall changes in hurricane strength or frequency over time.

Ultimately, however, this claim regarding hurricanes represents a straw man, as the climate research community does not currently claim that observed trends in cyclone frequency or intensity are attributable to climate change, and places low confidence on future projections of increased tropical cyclone strength (and decreased tropical cyclone frequency) in response to climate change (IPCC SROCC Ch 6).

That said, our ability to confidently attribute increased impacts from individual extreme events as a consequence of climate change has increased. For instance, researchers are able to conclusively state that the impact of Supertyphoon Haiyan in November of 2013 was exacerbated as a result of sea level rise (Trenberth et al., 2015).

Claim 11: The “most damaging” hurricane in US history occurred in 1900 in Galveston, Texas, so hurricanes cannot be getting more intense and destructive. People are also moving to the coasts, complicating interpretations of damages over time.

Response: Changing coastal infrastructure and patterns of human habitation over history work both ways. The deadliness of the Galveston hurricane can be attributed not just to its strength but also to the absence of an early-warning weather system, differences in construction methods, and more primitive organization of disaster response.

As for economic damage, the scientific literature is currently inconclusive regarding whether or not US hurricanes are inflicting more economic losses today than they did in the past (e.g. Estrada et al., 2015; Weinkle et al., 2018).

Claim 12: The media overstates the connection between climate change and tornadoes.

Response: The article utilized to represent this viewpoint explicitly states: “The scientific evidence is not strong enough for a definitive link between global warming and the kinds of severe thunderstorms that produce tornadoes.” Manheimer misrepresents its true position, which emphasizes uncertainty is in quite in line with current research regarding the lack of conclusive evidence for a connection between climate change and tornadoes (Hausfather, 2019).

Claim 13: California droughts are often held up as a symptom of climate change by TV news anchors. However, the overall drought severity index for the entire contiguous 48 states has not shown a trend since the pre-industrial era.

Response: Any earth scientist would of course frown at using a country-wide metric to refute a claim that a particular region is experiencing changes over time. Examining the same Palmer drought index for the southwestern United States region only (PAGES Hydro2k Consortium, 2017), we observe that contrary to Manheimer’s claim, a sharp shift in conditions towards a progressively drier, more drought-prone state has taken place over the observational record

Manheimer’s plot showing an overall flat trend in the drought index for the contiguous United States is unsurprising, as climate shifts have strengthened rainfall for some locations while reducing precipitation in other areas. While the southwestern United States, parts of the Mountain states, and portions of the southeastern US have experienced less rain over time, rainfall over the Midwest and Northeast has increased (USGCRP, 2018).

Claim 14: Agriculture is portrayed to be under threat from climate change, despite the fact that agricultural yields have increased over the late 20th century.

Response: To date, the planet has already warmed relative to the pre-industrial era by a global mean average temperature increase of 1C. Manheimer forgets that over the same period - indeed, during his own lifetime - a number of revolutionary technological changes have dramatically altered agriculture throughout much of the world. These included greatly expanded use of inorganic fertilizers, the breeding of vastly more productive staple crop variants, and the increasing mechanization and industrialization of agricultural production, collectively often referred to as the Green Revolution (Warren, 1998; Evenson and Gollin, 2003; Pingali, 2012). These innovations significantly increased crop yields across many world regions and are responsible for averting several famines and helping reduce world hunger. This transformative evolution of modern agriculture entirely explains the substantial increase in crop production worldwide.

That the impact of climate change was insufficient to negate the effects of such revolutionary technological change should not come as the slightest surprise. Pointing at increasing crop yields does nothing to disprove climate change - one might easily make the counterclaim that in the absence of climate change agricultural yields would have increased beyond the rate historically observed. Indeed, a sizable body of literature points towards negative impacts of climate change upon agricultural productivity (e.g. Chen et al., 2016; Moore et al., 2015).

Claim 15: CO2 may actually be beneficial, thanks to increasing agricultural productivity via CO2 fertilization.

Response: While increased CO2 concentrations can provide a beneficial fertilizing effect for some plants when CO2 levels adjusted in isolation, elevated atmospheric CO2 levels are of course accompanied by changes to temperature and precipitation patterns. Furthermore, not all crops are anticipated to benefit from CO2 fertilization. Agricultural plants utilizing the C4 photosynthetic pathway, such as corn and sugarcane, see few gains from increased CO2 levels (Cure et al., 1986; Leakey et al., 2006). Furthermore, when considered in conjunction with climatic changes, plant biomass gains from CO2 fertilization can be wiped out (Zhu et al., 2016). Ultimately, the impact of increased heat stress (e.g. Liu et al., 2016; Hawkins et al., 2013) and changes to precipitation patterns (Rosenzweig et al., 2002) is anticipated to outweigh any benefits provided by increased CO2 availability.

Finally, increasing concern over the future global food supply is not limited to the substantial worry regarding climate impacts (Hanjra et al., 2010) but further involves challenges presented by population growth and by the potential of a future flattening of growth in agricultural yields (Connor and Mínguez, 2012). The question of future food security for the world should not be dismissed so lightly.

Claim 16: Siberia is cooling very significantly. Look at this picture of Yakutsk, Siberia in winter, taken during an evening with temperatures of 60 degrees below zero!

Response: The choice in particular of Yakutsk in the Russian Federation is conspicuous, as it happens to be one of the coldest locations on Earth outside of Antarctica, and is popularly known as the world’s coldest major city. The lowest temperature ever reliably recorded apart from Antarctic measurements was taken nearby in the city of Oymyakon ~425 miles to the northeast in the same province of Siberia (Stepanova, 2015).

Manheimer presents no other proof that Siberia is purportedly cooling apart from a link to an EOS.org article taken out of context. This article reports on a study assessing a trend in which Siberian winter temperatures have been lower over the past several decades. Manheimer neglects to mention that lower Siberian winter temperatures are assessed by the study to result from changes in atmospheric circulation caused by increased melt of sea ice in the Barents and Kara Seas during the fall (Zhang et al., 2018).

With that aside, let us assess the trend in Siberian mean temperatures over the past century or more. Below, we present the long-term temperature data for four Russian meteorological stations in East Siberia (Desyatkin et al., 2015). All four time series show a clear increase in temperature since the 1950s.

Claim 17: Temperature records taken at the poles show only slightly increasing temperature for North Pole region and no trend at the South Pole region.

Response: The UAH MSU dataset Manheimer uses reports tropospheric temperatures, not surface temperatures, and furthermore possesses little coverage for latitudes between 80N-90N and 80S-90S. As the troposphere warms at a much slower rate than the Earth’s surface over the poles, the North Pole region plot of tropospheric temperatures shown by Manheimer does not capture the dramatic rate of increase in Arctic surface temperatures over the observational record (Overland et al., 2019), with sea surface temperatures in the region climbing at a pace of up to 0.5C per decade (IPCC SROCC Ch 3)

Manheimer is correct that the South Pole exhibits no significant long-term trend in temperature, a finding supported by weather station measurements from the Amundsen-Scott outpost over 54 years (Lazzara et al., 2012). This comes as no surprise to the earth science community, for which this result - as well as its cause - has been known for decades (Thompson et al., 2002; Turner et al., 2005). Portions of Antarctica’s interior are insulated from wider global temperature increases due to atmospheric and oceanic circulation patterns, and an increasing trend in the strength of circumpolar westerly winds caused by ozone depletion in the stratosphere of the Southern Hemisphere has been implicated as the cause of regional Antarctic cooling (Thompson et al., 2002).

Claim 18: Cutting CO2 threatens the lifestyles of billions.

Response: Manheimer commits a false dichotomy in implying that reducing anthropogenic greenhouse gas emissions necessarily requires substantial reductions in quality of life for billions of people. To offer some obvious counterexamples, if cheap fusion-based generation of electricity to be commercially demonstrated tomorrow, or if dramatic technological advances substantially lower the cost of atmospheric carbon removal while increasing its efficiency, it stands to reason that significant reductions in net CO2 emissions could be attained with no changes to standard of living. With ongoing improvements in the cost-competitiveness, efficiency, and scaleability of clean energy, energy storage, clean vehicles, one has little reason to expect that the burning of fossil fuels is inextricably tied to human well-being. Rather, a spectrum of solutions and policy approaches may permit humankind to largely decouple modern society from carbon emissions even as it continues to improve standards of living for people globally.

Watching the media take new findings and draw the wrong conclusions in science reporting is an experience that scientists across all disciplines are likely intimately familiar with. Writers might tend towards exaggerating the risks of generating a black hole with a particle accelerator or overestimating the ease of obtaining weapons-grade material from nuclear waste. Similarly, journalists and reporters have been and should be criticized for misinterpreting climate science in some cases, but the most appropriate response to those lapses merely requires scientists reaching out to correct the record on how some climate projections or impacts are framed or interpreted. The scientific basis behind anthropogenic climate change has grown far too robust for such minor misunderstandings to affect the larger pattern of a warming planet and evolving climate.

To focus overly on relatively insignificant popular misperceptions misses the bigger picture - namely, that the strong evidentiary basis for anthropogenic climate change warns us of serious ongoing and imminent consequences for human societies worldwide should our destabilizing perturbations of the earth system continue unabated.

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