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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.
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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.
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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.
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Quick filter by anthropogenic evidence type:
Shortlist Category Number Citations Year Cite As DOI Key Quote
Radiative GHE Observed 832001Top (Harries et al., 2001)https://doi.org/10.1038/35066553Here we analyse the difference between the spectra of the outgoing longwave radiation of the Earth as measured by orbiting spacecraft in 1970 and 1997. We find differences in the spectra that point to long-term changes in atmospheric CH4, CO2 and O3 as well as CFC-11 and CFC-12. Our results provide direct experimental evidence for a significant increase in the Earth's greenhouse effect that is consistent with concerns over radiative forcing of climate.
Radiative GHE Observed 562015Top (Feldman et al., 2015)https://doi.org/10.1038/nature14240Here we present observationally based evidence of clear-sky CO2 surface radiative forcing that is directly attributable to the increase, between 2000 and 2010, of 22 parts per million atmospheric CO2
Radiative GHE Observed 412004Top (Philipona et al., 2004)https://doi.org/10.1029/2003GL018765The resulting temperature corrected cloud-free longwave downward radiation (LDRcf,tc) shown in Figure 3b) has, due to the high correlation, now much less variability and a quite uniform increase between 2.1 and 2.9 Wm−2 over all stations. The average LDRcf,tc increase is +2.4 (0.9) Wm−2 and most of the stations show a trend at the 95% significance level.

The resulting uniform increase of longwave downward radiation manifests radiative forcing that is induced by increased greenhouse gas concentrations and water vapor feedback, and proves the -theory- of greenhouse warming with direct observations.
Radiative GHE Observed 442009(Wang and Liang, 2009)https://doi.org/10.1029/2009JD011800We found that daily Ld increased at an average rate of 2.2 W m^2 per decade from 1973 to 2008. The increase in Ld is mainly due to the increase in air temperature, water vapor and CO2 concentration.
Radiative GHE Observed 1262008(Wild, Grieser and Schär, 2008)https://doi.org/10.1029/2008GL034842Here we present a first-order trend estimate for the 15-year period 1986-2000, which suggests that surface net radiation over land has rapidly increased by about 2 Wm^2 per decade, after several decades with no evidence for an increase. This recent increase is caused by increases in both downward solar radiation (due to a more transparent atmosphere) and downward thermal radiation (due to enhanced concentrations of atmospheric greenhouse gases)
Radiative GHE Observed 152007(Prata, 2007)https://doi.org/10.1080/01431160802036508Here long-term (more than 25 years) mean monthly profiles obtained from globally distributed land-based radiosonde stations are subjected to detailed radiative transfer computations and Fourier time series analysis. The results indicate that over the period 1964-1990, there has been a global increase in the clear-sky longwave flux at the surface. The global trend is approximately +1.7 W m^2 per decade

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