The American public is poorly informed about the existence and primary cause of global warming. Just 67 percent of Americans are confident in the knowledge that global warming is happening, and a mere 53 percent understand that it’s mostly caused by human activities, while 33 percent of Americans continue to believe that global warming is mostly natural (Marlon et al. 2019). Of the latter, 13 percent believe that humans are not at all responsible for climate change, and a further 5 percent say the climate is not changing; this level of misconception is worse in the United States than any other developed country (Milman and Harvey 2019).

The situation is dramatically different among those who make a career of studying climate change; numerous studies using a variety of survey methods have found that there is a 90-to-100 percent expert consensus on human-caused global warming (Cook et al. 2016). This raises an oft-heard question, which can be loosely phrased as: How do the experts know that global warming is happening, and that humans are the cause? Herein is a summary of the evidence underlying what has been described as a “knowledge-based consensus” (Jacobs et al. 2018).

Evidence for global warming
We know that the planet is warming because of the data that has come in from a wide range of instrumental and natural thermometers over time.

Of these, the information that tends to be of most interest to humanity is the temperature of the air at the Earth’s surface, where people reside. In fact, thermometers have been used to measure local air temperatures since at least 1714, when German physicist Daniel Gabriel Fahrenheit invented the first accurate modern mercury thermometer. (His device—and the standardized scale of measurement that he devised to go with it—proved so popular that it led to the convention of temperatures being recorded in degrees Fahrenheit until the scientist Anders Celsius came along.)

As the saying goes, to someone with a hammer, everything is a nail—and the same could be said of those wielding a thermometer. As a result, there was an explosion in the collection of data regarding temperature, which drove 18th-century scientists such as geologist/physicist/mountaineer Horace Bénédict de Saussure to lead one of the first successful ascents of the tallest mountain in the Alps while equipped with a huge array of thermometers, altimeters, anemometers, magnetometers, barometers, microscopes, and all other manner of measuring devices. And it is important to note that these early scientists, just like their modern inheritors, did not go out into the field with the explicit goal of finding that the Earth was warming up. Rather, they had a passion for collecting data of all kinds, and let the information take them where it would.

This interest in measuring all the facets of our planet, over as long a span of time as possible, continues to this day; numerous scientific teams have used the available thermometer data to reconstruct average global surface temperatures (Lenssen et al. 2019, Morice et al. 2012, Vose et al. 2012, Cowtan and Way 2014) with the Berkeley Earth Surface Temperature team creating the longest such record, beginning in the year 1753 (Rohde et al. 2013). The records from all of these groups yield similar results: approximately 1 degree Celsius of average global surface temperature warming from the late-19th century through 2019.

But air temperatures represent a small fraction of the overall warming of Earth, over 90 percent of which is absorbed by the oceans.

Ocean temperatures have historically been measured from buckets on ships and recorded in naval logbooks (Drollette 2014), from instruments called bathythermographs, and since the early 21st century from a network of thousands of Argo floats (battery-powered autonomous floating devices that transmit ocean measurement data to satellites). These data indicate that Earth’s oceans have accumulated approximately 10 zettajoules of heat per year over the past 25 years (Cheng et al. 2020). This is equivalent to the amount of energy that would be released by detonating five Hiroshima atomic bombs per second (Nuccitelli 2020).

Climate scientists also monitor a variety of what are referred to as natural thermometers. For example, sea level rise is a consequence of both ocean warming (due to the thermal expansion of warmer water) and melting land ice. Global average sea level rise has been accelerating (Nerem et al. 2018) 2.5 times faster during 2006–2015 (about 3.6 millimeters per year) than 1901–1990 (1.4 millimeters per year). Half of recent sea level rise is due to melting land ice and 40 percent due to thermal expansion.

The accelerating rate is due to faster melt of the Greenland and Antarctic ice sheets. For the decade of 2007–2016, Greenland and Antarctic ice sheet loss doubled and tripled, respectively, compared to the prior decade (Oppenheimer et al. 2019). The melting of ice and the rising of sea levels are therefore both clear natural indicators, or natural thermometers, of global warming.

Although Arctic sea ice does not contribute to sea level rise (because unlike land ice, sea ice is already in the ocean), it has declined rapidly in recent decades. Primarily as a result of rising temperatures (Day et al. 2012), approximately 75 percent of the total quantity of summer Arctic sea ice has melted over the past 40 years. Shifts in the timing of the seasons (for example, spring beginning earlier) and changes in species migration patterns are both natural indicators and consequences of global warming (Bell et al. 2019, Diehl 2020).

Evidence of human causation
With these several independent observational lines of evidence, we come to the incontrovertible conclusion that the Earth is warming. Although a significant proportion of the American public remains unconvinced, nearly all climate science experts agree that this warming is predominantly human-caused (Cook et al. 2016). In its Fifth Assessment Report, the Intergovernmental Panel on Climate Change concluded with 95 percent confidence that humans are responsible for most of the global warming since 1950, and are most likely responsible for all of it (IPCC 2013). (This last part bears repeating: The report found that human beings are most likely responsible for all global warming since 1950.) As with the existence of global warming, its human cause has been established by multiple independent lines of evidence.

There are numerous fingerprints of the human-cause of global warming—such as changes in Earth’s climate that we expect to see as a consequence of the increased greenhouse effect, which has resulted from the emission of greenhouse gases, predominantly from the human burning of fossil fuels.

For example, because greenhouse gases are mostly present in the lower layer of the atmosphere (the troposphere) and trap outgoing heat at that elevation, climate scientists predicted over 50 years ago (Manabe and Wetherald 1967) that an increased greenhouse effect would result in a cooling of the higher layers of the atmosphere (that is, the stratosphere), which is indeed what was subsequently observed.

Long-term trends (either warming or cooling) in global temperatures are indicative of a global energy imbalance. A rise in temperatures over several decades is a result of more incoming than outgoing energy in Earth’s climate system, while falling temperatures are a consequence of more outgoing energy. An increased greenhouse effect will result in more trapped heat and consequently less outgoing energy from Earth, which has been confirmed by satellite observations.

Scientists have also used climate models to run “detection and attribution studies,” which seek to determine what physical causes can explain the observed global warming. In every case, these studies have concluded that human activity is responsible for approximately all of the warming observed since 1950 (Tett et al. 2002, Meehl et al. 2004, Stone et al. 2007, Lean and Rind 2008, Huber and Knutti 2012, Gillett et al. 2012, Wigley and Santer 2013, Jones et al. 2013, Ribes et al. 2017).

Ultimately, the attribution of global warming to human activities boils down to basic physics. We know that humans are responsible for the rapid increase in atmospheric greenhouse gas concentrations (carbon dioxide levels have risen nearly 50 percent since pre-industrial times). This has been established by examining isotope ratios—plants prefer lighter carbon-12 to the  heavier carbon-13, and fossil fuels are derived from ancient plants, so the ratio of carbon-13 to carbon-12 in the atmosphere has declined as more fossil fuel carbon has been added. It has also been established by simple accounting (Starr 2016)—scientists track the amount carbon released by the fossil fuels humans burn, and that carbon has to go somewhere. The increase of carbon in the oceans and biosphere and atmosphere is consistent with the amount humans are releasing from fossil fuels. And we’ve known for over a century that greenhouse gases trap heat and cause Earth to warm (Arrhenius 1896).

The rate of warming over the past century has also been much more rapid than historical natural climate changes. For example, during the transition 12,000 years ago from the last ice age to the current warm interglacial period, global temperatures warmed about 4-to-6 degrees Celsius over a period of about 10,000 years. In contrast, humans have caused 1 degree Celsius of warming over the past 100 years, and could trigger anywhere from another 1-to-4 degrees Celsius warming over the next 80 years (Clark et al. 2016). In short, the warming to date is already about 20 times faster than one of Earth’s fastest natural climate change events.

What about alternative causes?
When seeking alternative explanations for the primary cause of global warming, the sun is most often cited. After all, nearly all the heat on Earth originates from the sun, so it would make sense for rising temperatures to be caused by an increase in solar activity. But observations of sunspots and from satellites have shown that total solar irradiance (the amount of energy from the sun reaching Earth) has remained flat since 1950 (Kopp et al. 2016, Wu et al. 2018). Moreover, solar warming would cause the temperature of all layers of Earth’s atmosphere to rise, whereas as noted above, the upper atmospheric layers have cooled, inconsistent with a solar warming temperature profile, but consistent with an increased greenhouse effect.

Ocean cycles are another frequent suspect blamed for global warming. However, these cycles simply move heat back and forth between the oceans and atmosphere. Were ocean cycles to blame for rising air temperatures, heat would be transferred from the oceans to the atmosphere, and ocean temperatures would fall as a result. Instead, observations indicate that both the oceans and air are warming rapidly, which indicates that an influence creating a global energy imbalance must be responsible rather than an internal cycle.

Earth’s orbital cycles (Milankovitch cycles) around the sun are another possible culprit for global warming. However, these orbital cycles act over long timeframes, causing global temperatures to slowly warm or cool over thousands of years—whereas temperatures have risen a full degree Celsius just over the past century. Moreover, Earth has been in a slow cooling phase of these orbital cycles for the past several thousand years (Marsicek et al. 2018).

What the evidence shows
The conclusions that climate scientists draw are elementary.

Multiple independent lines of evidence show that the Earth is warming. These include instrumental measurements of rising air and ocean temperatures, as well as so-called “natural thermometers” such as melting ice, rising sea levels, and the shifting of seasons and animal migration patterns. Similarly, multiple independent lines of evidence tell us that humans are responsible for approximately all the global warming over the past 70 years. We know this through the fingerprints that humanity has left on the planet, in the form of the warming of the oceans and the lower atmosphere as the upper atmosphere cools, for example. This observed warming can only be explained by climate models that use the well-understood physics of the greenhouse effect—an effect that can only be explained as the result of human intervention, in the form of greenhouse gases produced by the consumption of fossil fuels.

Conversely, no alternative cause has been proposed that can explain all of these observations. Solar activity has been flat for seven decades, and had solar output increased, it would have warmed the upper atmosphere—which has actually gone down in temperature. If ocean cycles were responsible for warming the lower atmosphere, ocean temperatures would be cooling rather than heating at a rate equivalent to the amount of energy that would be released by detonating five Hiroshima atomic bombs per second, or every person on Earth constantly running 35 microwave ovens. Earth’s orbital cycles around the sun are in a gradual cooling phase, which in any case operate on much longer timescales than the abrupt global warming that has occurred over the past century.

Consequently, the scientific evidence clearly demonstrates that Earth is warming rapidly due to humans releasing tens of billions of tons of carbon dioxide into the atmosphere every year through the burning of fossil fuels.

Disclosure Statement
No potential conflict of interest was reported by the author.

Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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