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All Observed Anthropogenic:

• Represent a direct or indirect observation demonstrating why the warming trend since the mid-20th century is anthropogenic (i.e. the result of an enhanced greenhouse effect.)
• Are built upon an established framework of several assumed facts:
• That greenhouse gases are rising.
• That this rise is due to anthropogenic activities.
• That the planet has warmed since the mid-20th Century.
• That the presence of greenhouse gases in the atmosphere will (at least theoretically) cause a planet's temperature at its surface and lower atmosphere to be greater than if they were not present.
As an established baseline, the sources below are not focused on establishing the validity on any of the above given facts. They are instead focused specifically on the lines of evidence which attribute the observed temperature rise since the mid-20th century to this human-driven rise in greenhouse gases.

Decreasing DTR

Diurnal Temperature Range (DTR)

A shrinking diurnal temperature range (the difference between the hottest [daytime] and coldest [nighttime]...

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A shrinking diurnal temperature range (the difference between the hottest [daytime] and coldest [nighttime] temperature at a particular location) is a predicted effect of warming as a result of a strengthening greenhouse effect.

This can be intuitive. Consider that the majority of the radiation that interacts with greenhouse gases comes from the Earth's surface, as longwave infrared, rather than shortwave from the sun. Since the Earth's surface is emitting longwave IR day and night, the greenhouse effect works both day and night at any particular location, while the sun's energy only makes a contribution during the day.

Imagine a hypothetical, very weak, greenhouse effect. Take solar energy to be 1300 at high noon, 0 at night, with a greenhouse effect of 1 all the time. From day:night then, the planet receives 1301x more energy during the day than at night.

Now, increase the strength of the greenhouse effect to 500. During the day the combined incoming energy to the surface becomes (1301 + 500), compared to 500 at night. Now, the difference between day and night is only 3.6x. Both daytime and nighttime temperatures are higher with the stronger greenhouse effect, but it is the night that has gained by far the most proportionally, and this is true regardless of what the actual greenhouse gas contributions may be. It should be clear from this example too that increasing the solar component alone (either by increasing the sun's output, reducing cloud cover, or other global brightening factor) would *increase* the ratio from day to night by warming only daytime temperatures and keeping nighttime unaffected, thus producing the opposite signature of days warming faster than nights.

Astronomical analogies emphasize the point: Venus, with a greenhouse effect much stronger than Earth's, is a dramatic example of this effect, with a diurnal temperature range of effectively 0. Bodies with no greenhouse effect, however, such as the Moon and Mercury, have DTRs of approximately 300 C and 600 C, respectively.

Thus, a planetary warming driven by an enhanced greenhouse effect has a relatively unique expected signature: a multi-year warming rate that is greater at night than at daytime, or (in other words) an expected reduction in the mean difference between daytime maximum and nighttime minimum temperature. The multiple studies below which present evidence that this reduction is being observed thus represent a strong line of empirical evidence that the warming since the mid-20th century is being driven by an enhanced greenhouse effect.
Upper Atmosphere Cooling

Upper Atmosphere Cooling

The cooling of the upper atmosphere in response to an increase in CO2 was predicted over 50 years ago...

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The cooling of the upper atmosphere in response to an increase in CO2 was predicted over 50 years ago:

The larger the mixing ratio of carbon dioxide…
1) …the warmer is the equilibrium temperature of the earth's surface and troposphere.
2) …the colder is the equilibrium temperature of the stratosphere.

(Manabe and Wetherald, 1966; Journal of the Atmospheric Sciences, p251)

One reason for this is that greenhouse gases absorb infrared radiation that is emitted by the Earth’s surface; without them, infrared radiation emitted by Earth would proceed uninterrupted to space. As the concentration of greenhouse gases increase, any particular photon has a greater chance of being absorbed by a greenhouse gas molecule. Since a greenhouse gas molecule which absorbed the photon can re-radiate it in a random direction, this has the immediate effect of preventing its energy from reaching the upper atmosphere. Until equilibrium is re-established by surface warming (which generates more IR photons to compensate), the stratosphere receives less energy with an increase in the strength of the greenhouse effect in the troposphere, and therefore cools.

It is not necessary for a lack of equilibrium to exist, however. Consider that CO2 is less abundant in the stratosphere than it is in the troposphere. Put another way, more of the stratosphere is composed of gases that are not greenhouse gases, like O2 and N2, than it is by greenhouse gasses. Although N2 and O2 do not absorb longwave IR, they are able to absorb shorter wavelengths (from the sun). This increases their energy and they move faster, which is synonymous with being warmer. Because these molecules cannot emit longwave IR, energy transfer between molecules can only happen s via collisions (conduction). When an O2 or N2 molecule collides with another O2 or N2 molecule, kinetic energy can be transferred from one to the other.

CO2 is also capable of moving, but it is also able to vibrate in an excited state when it absorbs energy. When an N2 or O2 collides with a CO2 molecule, then, the N2/O2 will cool after transferring its energy to the CO2. But if the collision results not in a 1:1 transfer of kinetic energy, and instead the CO2 molecule moves to an excited vibrational state, kinetic energy (temperature) is no longer conserved and thus the average temperature of the collided molecules is less. The excited state is then relaxed by the emission of an IR photon. And because of the lower concentration of greenhouse gases in the upper atmosphere, any emitted photon has a greater chance of reaching space than would a photon emitted by a CO2 molecule in the troposphere.

Increasing CO2 concentrations in the troposphere will also increase them in the stratosphere, wherein they enable a more effective radiation of the photons, and thus lead to a cooler temperature.

By contrast, a warming caused by an increase in solar forcing, for example – either directly from the sun being more active, or from an increase in clouds (which would reflect shortwave, rather than longwave, back to the upper atmosphere) – would increase the shortwave absorption by N2 and O2 and therefore warm the stratosphere. Only a greenhouse-gas induced warming produces the signature of a warming lower atmosphere and cooling upper atmosphere.
Warming Anomalous Historically

Warming Anomalous Historically

In general, any trend in Earth’s temperature has an underlying physical cause. Without a driving, or forcing, of temperature change…

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In general, any trend in Earth’s temperature has an underlying physical cause. Without a driving, or forcing, of temperature change, the temperature would remain constant. There can be multiple possible causes of any temperature change, and so two different temperature changes in history – even of similar rate and magnitude – may not necessarily be due to the same underlying cause. Because of this, there is no reason to demand perfectly stable temperatures – or no record of similar temperature change events – in Earth’s geological past as a pre-requisite to attributing the current warming to man-made causes, if the causes underlying previous warming events are known to be nonexistent today.

In other words, the fact that the Earth's temperature *can* change naturally is not evidence that it can only ever change naturally. This would be no more logical than observing an animal dying naturally and concluding that humans are incapable of killing an animal. The question, therefore, is not whether the Earth's average temperature can vary naturally, because it certainly can: it is whether the observed increase in temperature since the mid-20th century is *also* natural.

What the recent stability of temperatures does do is show that the rate of warming observed since the mid-20th century has not been observed in the past 2,000 years and therefore any ‘natural’ cycle that could produce it must have a frequency that is longer than this. It also shows that something must have changed since the mid-20th century. Indeed, the overall trend from the proxy records tends to show that over many millenia the long-term trend was one of gradual cooling. A forcing must have been introduced that was not only not naturally present over the past 2,000 years, but which was sufficient to reverse the underlying cooling trend.

What this does not do is prove that the warming since the mid-20th century is due to human activities – this is only able to be demonstrated by ruling out other possible natural causes, and finding unique fingerprints in the way in which the planet is warming that are exclusively consistent with a warming caused by an increase in greenhouse gas concentrations. These fingerprints are presented as the other lines of evidence here.

Simplified figure originally from Mann et al., 2008, reproduced under educational use license. Copyright 2008 National Academy of Sciences.

Quick filter by anthropogenic evidence type:
Shortlist Category Number Citations Year Cite As DOI Key Quote
Decreasing DTR 23152006Top (Alexander et al., 2006) in the gridded fields were computed and tested for statistical significance. Results showed widespread significant changes in temperature extremes associated with warming, especially for those indices derived from daily minimum temperature...Daily maximum temperature indices showed similar changes but with smaller magnitudes.
Decreasing DTR 1522016Top (Davy et al., 2016), we review the observed diurnal asymmetry in the global warming trend: the night-time temperatures have increased more rapidly than day-time temperatures...we demonstrate that the night-time temperatures are inherently more sensitive to perturbations to the radiation balance and will warm more rapidly on a uniform forcing (such as that from the build-up of greenhouse-gases).
Decreasing DTR 4702005(Vose et al., 2005) data acquisitions are used to examine recent global trends in maximum temperature, minimum temperature, and the diurnal temperature range (DTR)...minimum temperature increased more rapidly than maximum temperature (0.204 vs. 0.141 C dec−1) from 1950-2004, resulting in a significant DTR decrease (−0.066 C dec−1).
Decreasing DTR 331995(Plummer et al., 1995) in maximum and minimum temperatures over Australia from 1951 to 1992 have been examined using data adjusted for inhomogeneities....showing a decrease in the diurnal temperature range (DTR) over large areas of the Australian continent...largely a consequence of the minimum temperatures increasing more than the maximum temperatures
Decreasing DTR 1572009(Choi et al., 2009) this study, spatial and temporal patterns of changes in extreme events of temperature and precipitation at 143 weather stations in ten Asia-Pacific Network (APN) countries and their associations with changes in climate means are examined for the 1955-2007 period. Averaged over the APN region, annual frequency of cool nights (days) has decreased by 6.4 days/decade (3.3 days/decade), whereas the frequency of warm nights (days) has increased by 5.4 days/decade (3.9 days/decade)
Decreasing DTR 3252006(Klein Tank et al., 2006) smaller warming of daytime versus nighttime extremes is consistent with the observed decrease in diurnal temperature range (DTR) shown in Figure 4 (0.12C/decade).
Decreasing DTR 1912013(Skansi et al., 2013) (minimum) temperature indices show the largest rates of warming (e.g. for tropical nights, cold and warm nights), while daytime (maximum) temperature indices also point to warming (e.g. for cold days, summer days, the annual lowest daytime temperature), but at lower rates than for minimums.
Decreasing DTR 10981997Top (Easterling et al., 1997) of the global mean surface air temperature has shown that its increase is due, at least in part, to differential changes in daily maximum and minimum temperatures, resulting in a narrowing of the diurnal temperature range (DTR)
Decreasing DTR 7201993(Karl et al., 1993) mean maximum and minimum temperatures for over 50% (10%) of the Northern (Southern) Hemisphere landmass, accounting for 37% of the global landmass, indicate that the rise of the minimum temperature has occurred at a rate three times that of the maximum temperature during the period 1951-90 (0.84C versus 0.28C)
Decreasing DTR 62008(Dufek et al., 2008;) over South America, the climate indices related to the minimum temperature (warm or cold nights) have clearly shown a warming tendency; however, no consistent changes in maximum temperature extremes (warm and cold days) have been observed
Decreasing DTR 521995(Horton, 1995) trends of maximum and minimum temperature, and diurnal range, are presented, revealing a marked decrease in diurnal temperature range in 1981-1990 compared with 1951-1980 over a large area of the world.
Decreasing DTR 3071991(Karl et al., 1991) large countries in the Northern Hemisphere (the contiguous United States, the Soviet Union, and the People's Republic of China)...most of the warming which has occurred in these regions over the past four decades can be attributed to an increase of mean minimum (mostly nighttime) temperatures. Mean maximum (mostly daytime) temperatures display little or no warming.
Decreasing DTR 232008(Soltani and Soltani, 2008) temperature (0.29°C/decade) at Bojnord and both minimum (0.45°C/decade) and maximum (0.24°C/decade) temperatures at Mashhad indicated significant positive trend. The positive trend for minimum temperature at Bojnord and Mashhad could be ascribed to greenhouse effect.
Decreasing DTR 1092016(Cao et al., 2016) on the new homogenized data set, linear trends in the annual and seasonal temperature series from 1960 to 2014 were calculated...The updated nationwide mean warming rate reached 0.22 C per 10 years for the Tmax...and 0.38 C per 10 years for the Tmin.
Decreasing DTR 992018(Tong et al., 2018) and spatial variations in extreme temperature and precipitation events in Inner Mongolia were analyzed...nighttime warming was higher than daytime warming.
Decreasing DTR 312019(Fallah-Ghalhari et al., 2019) this paper, trends of minimum and maximum temperatures in Iran were studied...the slope of the trend line for maximum and minimum temperature was obtained 0.23 and 0.39 C decade -1, respectively.
Decreasing DTR 82019(Liu et al., 2019) the Yarlung Tsangpo River Basin (1970-2017)...increases in daily minimum and maximum temperature were detected, and the magnitude of daily minimum temperature change was greater than that of the daily maximum temperature, revealing an obvious decrease in the diurnal temperature range.
Decreasing DTR 132020(Fathian et al., 2020) of 76 synoptic stations were selected for trend analysis throughout Iran during the period 1981-2010...the monthly maximum value of daily maximum (TXx) and minimum (TNx) temperatures increased by 0.3 and 0.4 C/decade on average, respectively; and the monthly minimum value of daily maximum (TXn) and minimum (TNn) temperatures increased by 0.1 and 0.3 C/decade, respectively
Decreasing DTR 1181998(Gaffen & Ross, 1998) the past half century, the mean summertime temperature in the United States has increased, with nights warming more than days.
Decreasing DTR 1122014(Stephenson et al., 2014) study presents the trends in daily and extreme temperature and precipitation indices in the Caribbean region for records spanning the 1961-2010 and 1986-2010 general, the indices based on minimum temperature show stronger warming trends than indices calculated from maximum temperature.
Decreasing DTR 92020(Feng et al., 2020) study investigated the spatial and temporal differences of compound hot and dry extremes over different regions and land covers in China during 1957-2018. On average over China, both day and night-time compound hot and dry extremes have increased with a more rapid (2.6 times faster) and significant (p < 0.05) increasing trend in night-time ones.
Decreasing DTR 102020(Qiu et al., 2020) 1967 to 2016 in China, Japan and the Philippines...greater increment in average daily minima in [daily minima] Ta (5.1%) was noted in the past 20 years compared to the average daily maximum in Ta (3.7%), showing asymmetric warming.
Decreasing DTR 991984(Karl et al., 1984)<14...An appreciable number of nonurban stations in the United States and Canada have been identified with statistically significant (at the 90% level) decreasing trends in the monthly mean diurnal temperature range between 1941-80.
Decreasing DTR 2462004(Braganza et al., 2004) reductions in DTR over the last century are large and unlikely to be due to natural variability alone.
Decreasing DTR 732009(Zhou et al., 2009) show that the surface diurnal temperature range (DTR) has decreased since 1950s over most global land areas due to a smaller warming in maximum temperatures (Tmax) than in minimum temperatures (Tmin)...when only natural forcings are used, none of the observed trends are simulated.
Decreasing DTR02022Top (Liu et al., 2022) indicate that the effect of anthropogenic forcing on the DTR is detectable separately from natural forcing across the globe and in many regions. GHG is the dominant contributor to DTR changes and caused the global DTR to decrease by -0.32 C during 1951-2018, close to the observed change of -0.41 C.
Decreasing DTR02022Top (Lu et al., 2022) multiple observational datasets, we find a general decrease in the DTR over most of the global land since 1901, especially after the mid-1950s.

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