How Have Humans Increased Global Warming
The extent of the homo contribution to mod global warming is a hotly debated topic in political circles, particularly in the United states of america.
During a recent congressional hearing, Rick Perry, the United states of america free energy secretary, remarked that "to stand up up and say that 100% of global warming is considering of human activity, I think on its face, is just indefensible".
However, the scientific discipline on the man contribution to modern warming is quite clear. Humans emissions and activities have caused around 100% of the warming observed since 1950, according to the Intergovernmental Panel on Climate Modify'southward (IPCC) 5th assessment written report.
Here Carbon Cursory examines how each of the major factors affecting the Globe's climate would influence temperatures in isolation – and how their combined effects almost perfectly predict long-term changes in the global temperature.
Carbon Brief's assay finds that:
- Since 1850, almost all the long-term warming can be explained by greenhouse gas emissions and other human activities.
- If greenhouse gas emissions lone were warming the planet, nosotros would expect to run across nigh a 3rd more than warming than has actually occurred. They are offset by cooling from human-produced atmospheric aerosols.
- Aerosols are projected to decline significantly by 2100, bringing total warming from all factors closer to warming from greenhouse gases alone.
- Natural variability in the Globe's climate is unlikely to play a major office in long-term warming.
How much warming is caused past humans?
In its 2022 fifth assessment report, the IPCC stated in its summary for policymakers that it is "extremely likely that more than than half of the observed increment in global average surface temperature" from 1951 to 2010 was acquired by human activity. By "extremely likely", information technology meant that there was betwixt a 95% and 100% probability that more than half of mod warming was due to humans.
This somewhat convoluted statement has been often misinterpreted every bit implying that the man responsibility for modernistic warming lies somewhere between 50% and 100%. In fact, as NASA's Dr Gavin Schmidt has pointed out, the IPCC's implied best approximate was that humans were responsible for around 110% of observed warming (ranging from 72% to 146%), with natural factors in isolation leading to a slight cooling over the by l years.
Similarly, the recent United states of america fourth national climate assessment plant that betwixt 93% to 123% of observed 1951-2010 warming was due to human activities.
These conclusions have led to some defoliation every bit to how more than 100% of observed warming could be attributable to human activeness. A homo contribution of greater than 100% is possible because natural climate change associated with volcanoes and solar action would most likely have resulted in a slight cooling over the by 50 years, offsetting some of the warming associated with human activities.
'Forcings' that change the climate
Scientists mensurate the various factors that affect the amount of energy that reaches and remains in the Earth's climate. They are known as "radiative forcings".
These forcings include greenhouse gases, which trap outgoing rut, aerosols – both from human activities and volcanic eruptions – that reflect incoming sunlight and influence cloud germination, changes in solar output, changes in the reflectivity of the Earth's surface associated with land use, and many other factors.
To assess the office of each different forcing in observed temperature changes, Carbon Brief adjusted a simple statistical climate model developed by Dr Karsten Haustein and his colleagues at the University of Oxford and University of Leeds. This model finds the human relationship between both human and natural climate forcings and temperature that best matches observed temperatures, both globally and over land areas only.
The figure below shows the estimated role of each different climate forcing in changing global surface temperatures since records began in 1850 – including greenhouse gases (scarlet line), aerosols (dark blueish), land use (light blueish), ozone (pinkish), solar (yellowish) and volcanoes (orangish).
The black dots show observed temperatures from the Berkeley World surface temperature project, while the grey line shows the estimated warming from the combination of all the dissimilar types of forcings.
Global mean surface temperatures from Berkeley Earth (black dots) and modeled influence of different radiative forcings (colored lines), besides equally the combination of all forcings (gray line) for the catamenia from 1850 to 2022. Run across methods at the end of the article for details. Chart by Carbon Brief using Highcharts.
The combination of all radiative forcings generally matches longer-term changes in observed temperatures quite well. There is some year-to-year variability, primarily from El Niño events, that is non driven by changes in forcings. In that location are besides periods from 1900-1920 and 1930-1950 where some larger disagreements are evident betwixt projected and observed warming, both in this simple model and in more complex climate models.
The nautical chart highlights that, of all the radiative forcings analysed, only increases in greenhouse gas emissions produce the magnitude of warming experienced over the past 150 years.
If greenhouse gas emissions alone were warming the planet, we would expect to run into near a third more than warming than has actually occurred.
And then, what roles practise all the other factors play?
The actress warming from greenhouse gases is being offset by sulphur dioxide and other products of fossil fuel combustion that course atmospheric aerosols. Aerosols in the atmosphere both reflect incoming solar radiation back into space and increase the formation of high, reflective clouds, cooling the Earth.
Ozone is a short-lived greenhouse gas that traps approachable heat and warms the Earth. Ozone is not emitted directly, merely is formed when methyl hydride, carbon monoxide, nitrogen oxides and volatile organic compounds break down in the atmosphere. Increases in ozone are straight attributable to human emissions of these gases.
In the upper atmosphere, reductions in ozone associated with chlorofluorocarbons (CFCs) and other halocarbons depleting the ozone layer have had a minor cooling effect. The net effects of combined lower and upper atmospheric ozone changes have modestly warmed the Earth by a few tenths of a degree.
Changes in the way country is used alter the reflectivity of the Earth's surface. For example, replacing a forest with a field volition generally increment the amount of sunlight reflected back into space, especially in snowy regions. The net climate outcome of land-use changes since 1850 is a modest cooling.
Volcanoes have a short-term cooling effect on the climate due to their injection of sulphate aerosols high into the stratosphere, where they can remain aloft for a few years, reflecting incoming sunlight back into space. Nevertheless, in one case the sulphates drift dorsum down to the surface, the cooling effect of volcanoes goes away. The orange line shows the estimated affect of volcanoes on the climate, with big downward spikes in temperatures of up to 0.4C associated with major eruptions.
Finally, solar activity is measured by satellites over the past few decades and estimated based on sunspot counts in the more distant past. The amount of energy reaching the Earth from the sun fluctuates modestly on a bike of around 11 years. There has been a slight increase in overall solar activity since the 1850s, but the amount of additional solar energy reaching the Earth is small compared to other radiative forcings examined.
Over the past 50 years, solar energy reaching the Earth has really declined slightly, while temperatures accept increased dramatically.
Human being forcings friction match observed warming
The accurateness of this model depends on the accuracy of the radiative forcing estimates. Some types of radiative forcing like that from atmospheric CO2 concentrations tin be direct measured and have relatively small uncertainties. Others, such as aerosols, are subject to much greater uncertainties due to the difficulty of accurately measuring their effects on cloud germination.
These are accounted for in the effigy below, which shows combined natural forcings (blue line) and human forcings (cerise line) and the uncertainties that the statistical model associates with each. These shaded areas are based on 200 different estimates of radiative forcings, incorporating enquiry attempting to estimate a range of values for each. Uncertainties in homo factors increase subsequently 1960, driven largely by increases in aerosol emissions after that point.
Global mean surface temperatures from Berkeley World (black dots) and modelled influence of all combined natural (blue line) and human (cherry-red line) radiative forcings with their corresponding uncertainties (shaded areas) for the period from 1850 to 2022. The combination of all natural and human forcings (grey line) is likewise shown. Run across methods at the finish of the article for details. Nautical chart past Carbon Brief using Highcharts.
Overall, warming associated with all human forcings agrees quite well with observed warming, showing that most 104% of the full since the outset of the "mod" period in 1950 comes from human being activities (and 103% since 1850), which is similar to the value reported past the IPCC. Combined natural forcings prove a modest cooling, primarily driven by volcanic eruptions.
The uncomplicated statistical model used for this analysis past Carbon Brief differs from much more circuitous climate models generally used by scientists to assess the human fingerprint on warming. Climate models do not simply "fit" forcings to observed temperatures. Climate models also include variations in temperature over infinite and time, and tin business relationship for unlike efficacies of radiative forcings in unlike regions of the Earth.
Even so, when analysing the impact of different forcings on global temperatures, complex climate models by and large find results similar to simple statistical models. The figure below, from the IPCC'due south Fifth Assessment Study, shows the influence of dissimilar factors on temperature for the menses from 1950 to 2010. Observed temperatures are shown in black, while the sum of human forcings is shown in orange.
This suggests that man forcings solitary would have resulted in approximately 110% of observed warming. The IPCC too included the estimated magnitude of internal variability over that flow in the models, which they suggest is relatively pocket-sized and comparable to that of natural forcings.
As Prof Gabi Hegerl at the University of Edinburgh tells Carbon Brief: "The IPCC report has an estimate that basically says the all-time guess is no contribution [from natural variability] with not that much uncertainty."
State areas are warming faster
Land temperatures have warmed considerably faster than average global temperatures over the by century, with temperatures reaching effectually 1.7C above pre-industrial levels in recent years. The state temperature tape also goes back further in time than the global temperature record, though the flow prior to 1850 is field of study to much greater uncertainties.
Both human and natural radiative forcings tin can be matched to state temperatures using the statistical model. The magnitude of homo and natural forcings volition differ a bit betwixt land and global temperatures. For example, volcanic eruptions announced to have a larger influence on state, equally land temperatures are probable to respond faster to rapid changes in forcings.
The figure below shows the relative contribution of each different radiative forcing to land temperatures since 1750.
Country hateful surface temperatures from Berkeley Earth (blackness dots) and modeled influence of different radiative forcings (colored lines), too as the combination of all forcings (grey line) for the period from 1750 to 2022. Chart by Carbon Cursory using Highcharts.
The combination of all forcings more often than not matches observed temperatures quite well, with brusque-term variability around the gray line primarily driven by El Niño and La Niña events. There is a wider variation in temperatures prior to 1850, reflecting the much larger uncertainties in the observational records that far dorsum.
There is still a flow around 1930 and 1940 where observations exceed what the model predicts, though the differences are less pronounced than in global temperatures and the 1900-1920 divergence is by and large absent in land records.
Volcanic eruptions in the late 1700s and early 1800s stand out sharply in the land tape. The eruption of Mount Tambora in Indonesia in 1815 may have cooled land temperatures past a massive 1.5C, though records at the fourth dimension were limited to parts of the Northern Hemisphere and it is, therefore, difficult to depict a business firm determination about global impacts. In full general, volcanoes appear to cool land temperatures by nearly twice as much as global temperatures.
What may happen in the future?
Carbon Brief used the same model to project future temperature changes associated with each forcing cistron. The figure beneath shows observations up to 2022, along with future post-2017 radiative forcings from RCP6.0, a medium-to-loftier future warming scenario.
Global mean surface temperatures from Berkeley Earth (blackness dots) and modeled influence of different radiative forcings (colored lines) for the flow from 1850 to 2100. Forcings post-2017 taken from RCP6.0. Chart by Carbon Brief using Highcharts.
When provided with the radiative forcings for the RCP6.0 scenario, the elementary statistical model shows warming of effectually 3C past 2100, virtually identical to the average warming that climate models observe.
Future radiative forcing from CO2 is expected to go on to increase if emissions ascension. Aerosols, on the other hand, are projected to peak at today's levels and decline significantly by 2100, driven in big part by concerns nigh air quality. This reduction in aerosols volition raise overall warming, bringing total warming from all radiative forcing closer to warming from greenhouse gases alone. The RCP scenarios assume no specific future volcanic eruptions, every bit the timing of these is unknowable, while solar output continues its 11-year cycle.
This approach can likewise be applied to land temperatures, equally shown in the figure below. Here, land temperatures are shown between 1750 and 2100, with post-2017 forcings also from RCP6.0.
Land mean surface temperatures from Berkeley World (blackness dots) and modeled influence of unlike radiative forcings (colored lines) for the period from 1750 to 2100. Forcings mail-2017 taken from RCP6.0. Chart by Carbon Cursory using Highcharts.
The country is expected to warm about 30% faster than the globe as a whole, every bit the charge per unit of warming over the oceans is buffered by body of water heat uptake. This is seen in the model results, where land warms by around 4C by 2100 compared to 3C globally in the RCP6.0 scenario.
At that place is a wide range of hereafter warming possible from unlike RCP scenarios and different values for the sensitivity of the climate arrangement, simply all evidence a like blueprint of declining futurity droplets emissions and a larger role for greenhouse gas forcing in futurity temperatures.
The role of natural variability
While natural forcings from solar and volcanoes practice not seem to play much of a office in long-term warming, there is also natural variability associated with bounding main cycles and variations in ocean heat uptake.
As the vast bulk of free energy trapped by greenhouse gases is absorbed by the oceans rather than the temper, changes in the rate of ocean heat uptake tin can potentially have big impacts on the surface temperature. Some researchers accept argued that multidecadal cycles, such as the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO), can play a role in warming at a decadal scale.
While human factors explain all the long-term warming, there are some specific periods that announced to take warmed or cooled faster than can be explained based on our all-time estimates of radiative forcing. For case, the modest mismatch between the radiative forcing-based estimate and observations during the mid-1900s might be evidence of a role for natural variability during that flow.
A number of researchers take examined the potential for natural variability to impact long-term warming trends. They have found that information technology more often than not plays a limited role. For example, Dr Markus Huber and Dr Reto Knutti at the Institute for Atmospheric and Climate Science (IAC) in Zurich found a maximum possible contribution of natural variability of around 26% (+/- 12%) over the by 100 years and 18% (+/- 9%) over the by 50 years.
Knutti tells Carbon Brief:
"We can never completely rule out that natural variability is larger than nosotros currently think. Merely that is a weak argument: you can, of course, never rule out the unknown unknown. The question is whether there is strong, or even whatsoever evidence for information technology. And the answer is no, in my view.
Models become the short-term temperature variability approximately right. In many cases, they even have likewise much. And for the long term, we tin't be sure because the observations are limited. But the forced response pretty much explains the observations, so there is no testify from the 20th century that we are missing something…
Even if models were found to underestimate internal variability by a factor of three, it is extremely unlikely [less than five% risk] that internal variability could produce a trend as large every bit observed."
Similarly, Dr Martin Stolpe and colleagues, also at IAC, recently analysed the role of multidecadal natural variability in both the Atlantic and Pacific oceans. They found that "less than 10% of the observed global warming during the second half of the 20th century is acquired by internal variability in these two bounding main basins, reinforcing the attribution of virtually of the observed warming to anthropogenic forcings".
Internal variability is likely to have a much larger function in regional temperatures. For example, in producing unusually warm periods in the Chill and the US in the 1930s. Nonetheless, its office in influencing long-term changes in global surface temperatures appears to be limited.
Conclusion
While there are natural factors that bear upon the Earth'south climate, the combined influence of volcanoes and changes in solar activity would have resulted in cooling rather than warming over the by fifty years.
The global warming witnessed over the by 150 years matches nearly perfectly what is expected from greenhouse gas emissions and other human activity, both in the simple model examined here and in more complex climate models. The best estimate of the human contribution to modern warming is effectually 100%.
Some uncertainty remains due to the role of natural variability, but researchers suggest that ocean fluctuations and similar factors are unlikely to exist the crusade of more than than a small fraction of modernistic global warming.
Methodology
The simple statistical model used in this commodity is adapted from the Global Warming Index published by Haustein et al (2017). In turn, it is based on the Otto et al (2015) model.
The model estimates contributions to observed climatic change and removes the impact of natural year-to-yr fluctuations by a multiple linear regression of observed temperatures and estimated responses to total human being-induced and total natural drivers of climatic change. The forcing responses are provided by the standard unproblematic climate model given in Chapter 8 of IPCC (2013), but the size of these responses is estimated by the fit to the observations. The forcings are based on IPCC (2013) values and were updated to 2022 using data from NOAA and ECLIPSE. 200 variations of these forcings were provided past Dr. Piers Forster of the University of Leeds, reflecting the uncertainty in forcing estimates. An Excel spreadsheet containing their model is also provided.
The model was adapted past computing forcing responses for each of the different major climate forcings rather than simply total man and natural forcings, using the Berkeley Earth record for observations. The decay time of thermal response used in converting forcings to forcing responses was adapted to exist one year rather than four years for volcanic forcings to improve reflect the fast response fourth dimension present in observations. The effects of El Niño and La Niña (ENSO) events was removed from the observations using an approach adjusted from Foster and Rahmstorf (2011) and the Kaplan El Niño 3.4 index when computing the volcanic temperature response, as the overlap between volcanoes and ENSO otherwise complicates empirical estimates.
The temperature response for each individual forcing was calculated by scaling their forcing responses by the total homo or natural coefficients from the regression model. The regression model was too run separately for land temperatures. Temperature responses for each forcing between 2022 and 2100 were estimated using forcing data from RCP6.0, normalised to friction match the magnitude of observed forcings at the finish of 2022.
Uncertainties in total human and full natural temperature response was estimated using a Monte Carlo analysis of 200 dissimilar forcing series, every bit well as the uncertainties in the estimated regression coefficients. The Python code used to run the model is archived with GitHub and bachelor for download.
Observational information from 2022 shown in the figures is based on the average of the first x months of the yr and is likely to exist quite similar to the ultimate annual value.
How Have Humans Increased Global Warming,
Source: https://www.carbonbrief.org/analysis-why-scientists-think-100-of-global-warming-is-due-to-humans
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