Is Human Activity Primarily Responsible for Global Climate Change?
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Last updated on: 4/1/2020 | Author: ProCon.org
Average surface temperatures on earth have risen more than 2°F over the past 100 years. During this time period, atmospheric levels of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) have notably increased. This site explores the debate on whether climate change is caused by humans (also known as anthropogenic climate change).
The pro side argues rising levels of atmospheric greenhouse gases are a direct result of human activities such as burning fossil fuels, and that these increases are causing significant and increasingly severe climate changes including global warming, loss of sea ice, sea level rise, stronger storms, and more droughts. They contend that immediate international action to reduce greenhouse gas emissions is necessary to prevent dire climate changes.
The con side argues human-generated greenhouse gas emissions are too small to substantially change the earth’s climate and that the planet is capable of absorbing those increases. They contend that warming over the 20th century resulted primarily from natural processes such as fluctuations in the sun’s heat and ocean currents. They say the theory of human-caused global climate change is based on questionable measurements, faulty climate models, and misleading science. Read more background…
Pro & Con Arguments
Pro 1
Overwhelming scientific consensus finds human activity primarily responsible for climate change.
According to many peer-reviewed studies, over 97% of climate scientists agree that human activity is extremely likely to be the cause of global climate change. [7] Most scientific organizations also support this view, including the American Medical Association and an international coalition of science academies. [7]
A prominent review of 11,944 peer-reviewed studies on climate change found that only 78 studies (0.7%) explicitly rejected the idea of anthropogenic (resulting from human activity) global warming. [1] A separate review of 13,950 peer-reviewed studies on climate change found only 24 that rejected human-caused global warming. [5] An examination of scientific papers that didn’t agree that humans cause climate change found serious flaws and bias in their research. [206]
Pro 2
Rising levels of human-produced gases released into the atmosphere create a greenhouse effect that traps heat and causes global warming.
Gases released into the atmosphere trap heat and cause the planet to warm through a process called the greenhouse effect. [8] When we burn fossil fuels to heat our homes, drive our cars, and run factories, we’re releasing emissions that cause the planet to warm. [9]
Methane, which is increasing in the atmosphere due to agriculture and fossil fuel production, traps 84 times as much heat as CO2 for the first 20 years it is in the atmosphere, [11] and is responsible for about one-fifth of global warming since 1750. [12] Nitrous oxide, primarily released through agricultural practices, traps 300 times as much heat as CO2. [13] Over the 20th century, as the concentrations of CO2, CH4, and NO2 increased in the atmosphere due to human activity, [13] [14] the earth warmed by approximately 1.4°F. [99]
Pro 3
The rise in atmospheric CO2 over the last century was clearly caused by human activity, as it occurred at a rate much faster than natural climate changes could produce.
Over the past 650,000 years, atmospheric CO2 levels did not rise above 300 ppm until the mid-20th century. [100] Atmospheric levels of CO2 have risen from about 317 ppm in 1958 to 415 ppm in 2019. [10] [194] According to the Scripps Institution of Oceanology, the “extreme speed at which carbon dioxide concentrations are increasing is unprecedented. An increase of 10 parts per million might have needed 1,000 years or more to come to pass during ancient climate change events.” [17] Some climate models predict that by the end of the 21st century an additional 5°F-10°F of warming will occur. [16]
Pro 4
The specific type of CO2 that is increasing in earth’s atmosphere can be directly connected to human activity.
We can tell that CO2 produced by humans burning fossil fuels such as oil and coal [18] is different than naturally occurring CO2 by looking at the specific isotopic ratio. [101] According to the Intergovernmental Panel on Climate Change (IPCC), 20th century measurements of CO2 isotope ratios in the atmosphere confirm that rising CO2 levels are the result of human activity as opposed to gas coming off the oceans, volcanic activity, or other natural causes. [102]
The US Environmental Protection Agency says that “Human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere over the last 150 years.” [19]
Pro 5
Average temperatures on earth have increased at a rate far faster than can be explained by natural climate changes.
Average surface temperatures on earth have risen more than 2°F over the past 100 years. [205] According to NASA, “The current warming trend is of particular significance because most of it is extremely likely (greater than 95 percent probability) to be the result of human activity since the mid-20th century and proceeding at a rate that is unprecedented over decades to millennia.” [24]
A 2008 study comparing data from tree rings, ice cores, and corals over the past millennium created the famous “hockey stick” graph showing a steady trend in the earth’s temperature over the last 1,700 years followed by a steep jump in the previous decade (forming a shape like a hockey stick). [23] Berkeley scientists found that the average temperature of the earth’s land increased 2.5°F over 250 years (1750-2000), 1.5°F of which “appears likely” to be attributable to humans over the past 50 years. [21]
Pro 6
Natural changes in the sun’s activity cannot explain 20th century global warming.
The amount of solar energy received by the earth goes up and down in cycles, but overall there is no net change since the 1950s. There has, however, been a big increase in global temperatures that is too large to attribute to the sun. For this reason, NASA and other scientists say the sun is not responsible for global warming. [28] The sun has had only a minor effect on the Northern Hemisphere climate over the past 1,000 years, and global warming from human-produced greenhouse gases has been the primary cause of climate change since 1900. [26] A study found that solar activity could not have contributed to more than 10% of the observed global warming over the 20th century. [27]
Pro 7
Global warming caused by human-produced greenhouse gases is causing the Arctic ice cap to melt at an increasing rate.
From 1953–2006, Arctic sea ice declined 7.8% per decade. Between 1979 and 2006, the decline was 9.1% each decade. [105] By 2019, Arctic sea ice was being lost at a rate of 12.9% per decade. [163] As the Arctic ice cover continues to decrease, the amount of the sun’s heat reflected by the ice back into space also decreases. This positive-feedback loop amplifies global warming at a rate even faster than previous climate models had predicted. [30] Some studies predicted the Arctic could become nearly ice free sometime between 2020-2060. [164]
Pro 8
Sea levels are rising at an unprecedented rate due to human activities.
Sea levels rise due to thermal expansion of warming ocean waters and melt water from receding glaciers and the polar ice cap. [165] According to the IPCC, there has been a “substantial” human contribution to the global mean sea-level rise since the 1970s. [29] As much as 87% of the rise in sea levels since 1970 resulted from human activities such as burning fossil fuels. [35]
A study found that “significant acceleration” of sea-level rise occurred from 1870 to 2004. [106] Between 1961 and 2003, global sea levels rose 8 inches; a 2019 UN report said they could rise by 3 feet in the next 80 years, displacing hundreds of millions of people. [102] [20] A study published in the Proceedings of the National Academy of Sciences concluded that the rate of sea level rise over the past century is unprecedented over the last 6,000 years. [32] [33]
Pro 9
Ocean acidity levels are increasing at an unprecedented rate that can only be explained by human activity.
As excess human-produced CO2 in the atmosphere is absorbed by the oceans, the acidity level of the water increases. Acidity levels in the oceans are 25-30% higher than prior to human fossil fuel use. [107] The US Government Accountability Office (GAO) said oceans have absorbed about 30% of the CO2 emitted by humans over the past 200 years, and ocean acidity could rise approximately 100-200 percent above preindustrial levels by 2100. [36]
The World Meteorological Organization said the current acceleration in the rate of ocean acidification “appears unprecedented” over the last 300 million years. [37] High ocean acidity levels threaten marine species, [16] and slows the growth of coral reefs. [38] The Convention on Biological Diversity said “it is now nearly inevitable” that within 50-100 years continued human-produced CO2 emissions will increase ocean acidity to levels that harm marine organisms and ecosystems. [39]
Pro 10
Ocean temperatures are rising at an unprecedented rate due to anthropogenic global warming.
Peter Gleckler, PhD, a climate scientist at Lawrence Livermore National Laboratory, said, “The bottom line is that… most of the observed global ocean warming over the past 50 years is attributable to human activities.” [42] The IPCC stated in a report that due to human-caused global warming, it is “virtually certain” (99-100% probability) that the upper ocean warmed between 1971 and 2010. [29] The oceans absorb more than 90% of the heat generated by human-caused global warming. [41] Since 1970, the upper ocean (above 700 meters) has been warming 24-55% faster than previous studies had predicted. [41]
Warmer ocean waters can harm coral reefs and impact many species including krill, which are vital to the marine food chain and which reproduce significantly less in warmer water. [166] Warming oceans also contribute to sea level rise due to thermal expansion, and can add to the intensity of storm systems. [167]
Pro 11
Glaciers are melting at unprecedented rates due to global warming, causing additional climate changes.
About a quarter of the globe’s glacial loss from 1851-2010, and approximately two thirds of glacial loss between 1991-2010, is attributable directly to global warming caused by human-produced greenhouse gases. [45] According to the National Snow and Ice Data Center, global warming from human-produced greenhouse gases is a primary cause of the “unprecedented” retreat of glaciers around the world since the early 20th century. [44]
Since 1980, glaciers worldwide have lost nearly 40 feet (12 meters) in average thickness. [110] According to an IPCC report, “glaciers have continued to shrink almost worldwide” over the prior two decades, and there is “high confidence” (about an 8 out of 10 chance) that Northern Hemisphere spring snow continues to decrease. [29] If the glaciers forming the Greenland ice sheet were to melt entirely, global sea levels could increase by up to 20 feet. [168]
Pro 12
Human-caused global warming is changing weather systems and making heat waves and droughts more intense and more frequent.
A National Climate Assessment report said human-caused climate changes, such as increased heat waves and drought, “are visible in every state.” [16] The American Meteorological Society found that anthropogenic climate change “greatly increased” (up to 10 times) the risk for extreme heat waves. [46] Globally, 75% of extremely hot days are attributable to warming caused by human activity. [174] A World Weather Attribution study found that anthropogenic climate change increased the likelihood of wildfires such as the ones that raged across Australia in 2019-2020 by at least 30% since 1900. [203]
Pro 13
Dramatic changes in precipitation, such as heavier storms and less snow, are another sign that humans are causing global climate change.
As human-produced greenhouse gases heat the planet, increased humidity (water vapor in the atmosphere) results. Water vapor is itself a greenhouse gas. [112] In a process known as a positive feedback loop, more warming causes more humidity which causes even more warming. [113] Higher humidity levels also cause changes in precipitation. According to a report published in the Proceedings of the National Academy of Sciences, the recorded changes in precipitation over land and oceans “are unlikely to arise purely due to natural climate variability.” [48]
According to researchers at the Scripps Institution of Oceanography, up to 60% of the changes in river flow, winter air temperature, and snow pack in the western United States (1950-1999) were human-induced. [111] Since 1991, heavy precipitation events have been 30% above the 1901-1960 average in the Northeast, Midwest, and upper Great Plains regions. [16] A study found that global warming caused by human actions has increased extreme precipitation events by 18% across the globe, and that if temperatures continue to rise an increase of 40% can be expected. [174]
Pro 14
Permafrost is melting at unprecedented rates due to global warming, causing further climate changes.
According to the IPCC, there is “high confidence” (about an 8 out of 10 chance) that anthropogenic global warming is causing permafrost, a subsurface layer of frozen soil, to melt in high-latitude regions and in high-elevation regions. [49] As permafrost melts it releases methane, a greenhouse gas that absorbs 84 times more heat than CO2 for the first 20 years it is in the atmosphere, creating even more global warming in a positive feedback loop. [50][51]
By the end of the 21st century, warming temperatures in the Arctic will cause a 30%-70% decline in permafrost. [52] As human-caused global warming continues, Arctic air temperatures are expected to increase at twice the global rate, increasing the rate of permafrost melt, changing the local hydrology, and impacting critical habitat for native species and migratory birds. [53] According to the 2014 National Climate Assessment, some climate models suggest that near-surface permafrost will be “lost entirely” from large parts of Alaska by the end of the 21st century. [16]
Con 1
Many scientists disagree that human activity is primarily responsible for global climate change.
A report found more than 1,000 scientists who disagreed that humans are primarily responsible for global climate change. [55] The claim that 97% of scientists agree on the cause of global warming is inaccurate. The research on 11,944 studies actually found that only 3,974 even expressed a view on the issue. Of those, just 64 (1.6%) said humans are the main cause. [54]
A Purdue University survey found that 47% of climatologists challenge the idea that humans are primarily responsible for climate change and instead believe that climate change is caused by an equal combination of humans and the environment (37%), mostly by the environment (5%), or that there’s not enough information to say (5%). [173]
Con 2
Earth’s climate has always warmed and cooled, and the 20th century rise in global temperature is within the bounds of natural temperature fluctuations over the past 3,000 years.
Although the planet has warmed 1-1.4°F over the 20th century, it is within the +/- 5°F range of the past 3,000 years. [114] A study by researchers at the Harvard-Smithsonian Center for Astrophysics found that “many records reveal that the 20th century is probably not the warmest nor a uniquely extreme climatic period of the last millennium.” [115]
A study published in Nature found that “high temperatures – similar to those observed in the twentieth century before 1990 – occurred around AD 1000 to 1100” in the Northern Hemisphere. [116] A study published in Boreas found that summer temperatures during the Roman Empire and Medieval periods were “consistently higher” than temperatures during the 20th century. [59]
Con 3
Rising levels of atmospheric CO2 do not necessarily cause global warming.
Earth’s climate record shows that warming has preceded, not followed, a rise in CO2. According to a study published in Science, measurements of ice core samples showed that over the last four climactic cycles (past 240,000 years), periods of natural global warming preceded global increases in CO2. [117] The Proceedings of the National Academy of Sciences published a study of the earth’s climate 460-445 million years ago which found that an intense period of glaciation, not warming, occurred when CO2 levels were 5 times higher than they are today. [4] According to ecologist and former Director of Greenpeace International Patrick Moore, PhD, “there is some correlation, but little evidence, to support a direct causal relationship between CO2 and global temperature through the millennia.” [60]
Con 4
Human-produced CO2 is re-absorbed by oceans, forests, and other “carbon sinks,” negating any climate changes.
A paper published in Asia-Pacific Journal of Atmospheric Sciences found that some climate models overstated how much warming would occur from additional C02 emissions. [75] About 50% of the CO2 released by the burning of fossil fuels and other human activities has already been re-absorbed by the earth’s carbon sinks. [118] From 2002-2011, 26% of human-caused CO2 emissions were absorbed specifically by the world’s oceans. [61] A study published in the Proceedings of the National Academy of Sciences found evidence that forests are increasing their growth rates in response to elevated levels of CO2, [62] which will in turn, lower atmospheric CO2 levels.
Con 5
CO2 is so saturated in earth’s atmosphere that more CO2, manmade or natural, will have little impact on the climate.
As CO2 levels in the atmosphere rise, the amount of additional warming caused by the increased concentration becomes less and less pronounced. [65] According to Senate testimony by William Happer, PhD, Professor of Physics at Princeton University, “[a]dditional increments of CO2 will cause relatively less direct warming because we already have so much CO2 in the atmosphere that it has blocked most of the infrared radiation that it can. The technical jargon for this is that the CO2 absorption band is nearly ‘saturated’ at current CO2 levels.” [66]
According to the Heartland Institute’s 2013 Nongovernmental International Panel on Climate Change (NIPCC) report, “it is likely rising atmospheric CO2 concentrations will have little impact on future climate.” [67]
Con 6
Global warming and cooling are primarily caused by fluctuations in the sun’s heat (solar forcing), not by human activity.
According to a study published in the Journal of Atmospheric and Solar-Terrestrial Physics, 50-70% of warming throughout the 20th century could be associated with an increased amount of solar activity. [71] Between 1900 and 2000 solar irradiance increased 0.19%, and correlated with the rise in US surface temperatures over the 20th century. [114]
A study published in Energy & Environment wrote, “variations in solar activity and not the burning of fossil fuels are the direct cause of the observed multiyear variations in climatic responses.” [69] In a study by Willie Soon, PhD, Physicist at the Harvard-Smithsonian Center for Astrophysics, a strong correlation between solar radiation and temperatures in the Arctic over the past 130 years was identified. [70]
Con 7
The rate of global warming has slowed over the last decade even though atmospheric CO2 continues to increase.
The Heartland Institute’s 2013 NIPCC report stated that the earth “has not warmed significantly for the past 16 years despite an 8% increase in atmospheric CO2.” [67] According to Emeritus Professor of Meteorology at the Massachusetts Institute of Technology Richard Lindzen, PhD, the IPCC’s “excuse for the absence of warming over the past 17 years is that the heat is hiding in the deep ocean. However, this is simply an admission that the [climate] models fail to simulate the exchanges of heat between the surface layers and the deeper oceans” [73]
Con 8
Sea levels have been steadily rising for thousands of years, and the increase has nothing to do with humans.
A report by the Global Warming Policy Foundation found that a slow global sea level rise has been ongoing for the last 10,000 years. [79] When the earth began coming out of the Pleistocene Ice Age 18,000 years ago, sea levels were about 400 feet lower than they are today and have been steadily rising ever since. [60]
According to Professor of Earth and Atmospheric Sciences at the Georgia Institute of Technology, Judith Curry, PhD, “it is clear that natural variability has dominated sea level rise during the 20th century, with changes in ocean heat content and changes in precipitation patterns.” [80]
Con 9
The acidity levels of the oceans are within past natural levels and the current rise in acidity is a natural fluctuation.
[120] The pH of average ocean surface water is 8.1 and has only decreased 0.1 since the beginning of the industrial revolution (neutral is pH 7, acid is below pH 7). [121] Science published a study of ocean acidity levels over the past 15 million years, finding that the “samples record surface seawater pH values that are within the range observed in the oceans today.” [82]
Increased atmospheric CO2 absorbed by the oceans results in higher rates of photosynthesis and faster growth of ocean plants and phytoplankton, which increases pH levels keeping the water alkaline, not acidic. [60] According to the Science and Public Policy Institute, “our harmless emissions of trifling quantities of carbon dioxide cannot possibly acidify the oceans.” [63]
Con 10
A lot of climate change fears are based on predictions and inadequate or flawed computer climate models.
Climate models have been unable to simulate major known features of past climate such as the ice ages or the very warm climates of the Miocene, Eocene, and Cretaceous periods. If models cannot replicate past climate changes they should not be trusted to predict future climate changes. [58] A Asia-Pacific Journal of Atmospheric Science study using observational data rather than computer climate models concluded that “the models are exaggerating climate sensitivity” and overestimate how fast the earth will warm as CO2 levels increase. [75]
Two other studies using observational data found that IPCC projections of future global warming are too high. [76] [97] Climatologist and former NASA scientist Roy Spencer, PhD, concluded that 95% of climate models have “over-forecast the warming trend since 1979.” [77] According to Emeritus Professor of Geography at the University of Winnipeg, Tim Ball, PhD, “IPCC computer climate models are the vehicles of deception… [T]hey create the results they are designed to produce.” [78]
Con 11
Glaciers have been growing and receding for thousands of years due to natural causes, not human activity.
The IPCC predicted that Himalayan glaciers would likely melt away by 2035, a prediction they disavowed in 2010. [83] In 2014 a study of study of 2,181 Himalayan glaciers from 2000-2011 showed that 86.6% of the glaciers were not receding. [84]
A study of ice cores published in Nature Geoscience said the current melting of glaciers in Western Antarctica was due to “atmospheric circulation changes” that have “caused rapid warming over the West Antarctic Ice Sheet” and cannot be directly attributed to human caused climate change. [85] According to one of the study authors, “[i]f we could look back at this region of Antarctica in the 1940s and 1830s, we would find that the regional climate would look a lot like it does today, and I think we also would find the glaciers retreating much as they are today.” [86] According to Christian Schlüchter, Professor of Geology at the University of Bern, the retreat of glaciers in the Alps began in the mid-19th century, before large amounts of human caused CO2 had entered the atmosphere. [87]
Con 12
Deep ocean currents, not human activity, are a primary driver of natural climate warming and cooling cycles.
Over the 20th century there have been two Arctic warming periods with a cooling period (1940-1970) in between. According to a study in Geophysical Research Letters, natural shifts in the ocean currents are the major cause of these climate changes, not human-generated greenhouse gases. [124] William Gray, PhD, Emeritus Professor of Atmospheric Science at Colorado State University, said most of the climate changes over the last century are natural and “due to multi-decadal and multi-century changes in deep global ocean currents.” [122]
Global cooling from 1940 to the 1970s, and warming from the 1970s to 2008, coincided with fluctuations in ocean currents and cloud cover driven by the Pacific Decadal Oscillation (PDO) – a naturally occurring rearrangement in atmospheric and oceanic circulation patterns. [123] According to Don Easterbrook, PhD, Professor Emeritus of Geology at Western Washington University, the “PDO cool mode has replaced the warm mode in the Pacific Ocean, virtually assuring us of about 30 years of global cooling, perhaps much deeper than the global cooling from about 1945 to 1977.” [88]
Con 13
Increased hurricane activity and other extreme weather events are a result of natural weather patterns, not human-caused climate change.
According to a report from the Tropical Meteorology Project at Colorado State University, the increase in human-produced CO2 over the past century has had “little or no significant effect” on global tropical cyclone activity. The report stated that specific hurricanes, including Sandy, Ivan, Katrina, Rita, Wilma, and Ike, were not a direct consequence of human-caused global warming. [89] Between 1995-2015, increased hurricane activity (including Katrina) was recorded; however, according to the NOAA, this was the result of cyclical tropical cyclone patterns driven primarily by natural ocean currents. [125]
Professor of Earth and Atmospheric Sciences at the Georgia Institute of Technology, Judith Curry, PhD, stated that she was “unconvinced by any of the arguments that I have seen that attributes a single extreme weather event, a cluster of extreme weather events, or statistics of extreme weather events” to human-caused climate change. [90] Experts have noted that many factors beyond climate change are to blame for events such as wildfires, including failed policies on clearing brush, too much population density, and people who set the fires either deliberately or through carelessness. [204]
Did You Know?
1. A 2019 report from the Yale Program on Climate Change Communication found that 73% of Americans think global warming is occurring, marking a ten percentage point increase over Mar. 2015. [183]
2. According to many peer-reviewed studies, over 97% of climate scientists agree that human activity is extremely likely to be the cause of global climate change. [7]
3. A peer-reviewed study found that "up to 70% of the observed post-1850 climate change and warming could be associated to multiple solar cycles." [2]
4. The Unites States makes up about 4% of the world’s population but was responsible for nearly one-third of historical global greenhouse gas emissions. [103]
5. A peer-reviewed study of the earth's climate 460-445 million years ago found that an intense period of glaciation, not warming, occurred when CO2 levels were 5 times higher than they are today. [4]
