climate
observed
 Home  Observed Warming  Observed Anthropogenic  Observed Effects

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]...

(Show More)
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.
(Collapse)
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...

(Show More)
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.
(Collapse)
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…

(Show More)
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.
(Collapse)

Quick filter by anthropogenic evidence type:
Shortlist Category Number Citations Year Cite As DOI Key Quote
Warming Anomalous Historically 252019Top (Neukom, R. et al., 2019)https://doi.org/10.1038/s41586-019-1401-2Here we use global palaeoclimate reconstructions for the past 2,000 years, and find no evidence for preindustrial globally coherent cold and warm epochs... This provides strong evidence that anthropogenic global warming is not only unparalleled in terms of absolute temperatures, but also unprecedented in spatial consistency within the context of the past 2,000 years.
Warming Anomalous Historically 92013(Ahmed et al., 2013)https://doi.org/10.1038/ngeo1834Recent warming reversed the long-term cooling; during the period AD 1971-2000, the area-weighted average reconstructed temperature was higher than any other time in nearly 1,400 years.
Warming Anomalous Historically 6362008(Mann et al., 2008)https://doi.org/10.1073/pnas.0805721105We find that the hemispheric-scale warmth of the past decade for the NH is likely anomalous in the context of not just the past 1,000 years, as suggested in previous work, but longer.
Warming Anomalous Historically 772007(Juckes et al., 2007)https://doi.org/10.5194/cp-3-591-2007A reconstruction using 13 proxy records extending back to AD 1000 shows a maximum pre-industrial temperature of 0.25 K (relative to the 1866 to 1970 mean)...Instrumental temperatures for two recent years (1998 and 2005) have exceeded the pre-industrial estimated maximum by more than 4 standard deviations of the calibration period residual.
Warming Anomalous Historically 562012(Leclercq and Oerlemans, 2012)https://doi.org/10.1007/s00382-011-1145-7The rate of temperature change over the period 1980-2000, with a linear trend of 0.16 K per decade, is the highest over the last 400 years...Our reconstruction supports the conclusion of Mann et al. (2008) that the high global average temperatures of the 1990-2000 decade are unprecedented in at least the last four centuries.
Warming Anomalous Historically 1612010(Ljungqvist, 2010)https://doi.org/10.1111/j.1468-0459.2010.0039...The temperature of the last two decades, however, is possibly higher than during any previous time in the past two millennia
Warming Anomalous Historically 8752009Top (Mann et al., 2009)https://doi.org/10.1126/science.1177303The Medieval period is found to display warmth that matches or exceeds that of the past decade in some regions, but which falls well below recent levels globally
Warming Anomalous Historically 2392000(Huang et al., 2000)https://doi.org/10.1038/35001556We use present-day temperatures in 616 boreholes from all continents except Antarctica to reconstruct century-long trends in temperatures over the past 500 years at global, hemispheric and continental scales. The results confirm the unusual warming of the twentieth century revealed by the instrumental record
Warming Anomalous Historically 8791999(Mann et al., 1999)https://doi.org/10.1029/1999GL900070our results suggest that the latter 20th century is anomalous in the context of at least the past millennium...20th century warming counters a millennial-scale cooling trend which is consistent with long-term astronomical forcing.
Warming Anomalous Historically 482011(Mann et al., 2011)https://doi.org/10.1029/2003EO270003A number of reconstructions of large-scale temperature changes over the past millennium support the conclusion that late-20th century warmth was unprecedented over at least the past millennium.
Warming Anomalous Historically 1521995(Briffa et al., 1995)https://doi.org/10.1038/376156a0Here we report a tree-ring-based reconstruction of mean summer temperatures over the northern Urals since AD 914. This record shows that the mean temperature of the twentieth century (1901-90) is higher than during any similar period since AD 914.
Warming Anomalous Historically 3282001(Briffa et al., 2001)https://doi.org/10.1029/2000JD900617We describe new reconstructions of northern extratropical summer temperatures for nine subcontinental?scale regions and a composite series representing quasi "Northern Hemisphere" temperature change over the last 600 years...the 20th century is clearly shown by all of the palaeoseries composites to be the warmest during this period.
Warming Anomalous Historically 3392003(Mann & Jones, 2003)https://doi.org/10.1029/2003GL017814We present reconstructions of Northern and Southern Hemisphere mean surface temperature over the past two millennia based on high?resolution ?proxy? temperature data which retain millennial?scale variability. These reconstructions indicate that late 20th century warmth is unprecedented for at least roughly the past two millennia for the Northern Hemisphere.
Warming Anomalous Historically 3052001(Jones et al., 2001)https://doi.org/10.1126/science.1059126Average temperatures during the last three decades were likely the warmest of the last millennium, about 0.2?C warmer than during warm periods in the 11th and 12th centuries. The 20th century experienced the strongest warming trend of the millennium (about 0.6?C per century)
Warming Anomalous Historically02020(Pang et al., 2020)https://doi.org/10.1029/2020JD032560Our reconstructed temperature record shows a long?term warming trend until ~2,000 years before present, followed by an abrupt change to a relatively cool period until the start of the industrial?era warming...our record shows that temperatures during the recent decades are almost the highest during the past 7,000 years, highlighting the unusual warming forced by anthropogenic greenhouse gases.

Select all results