A frequent argument made by people who are unconvinced that humans are causing global warming is that there's no evidence that the warming is anthropogenic (man-made). Many people will acknowledge that the planet is warming and that by increasing the amount of greenhouse gases in the atmosphere, humans are contributing to the warming to some degree. But, the skeptics argue, for all we know the human contribution is small.
As it turns out, there is overwhelming evidence that humans are causing the vast majority of the current global warming. For starters, the anthropogenic global warming theory is supported by solid fundamental physics, as discussed in the Global Warming Causes Wiki. However, there are also some very clear physical fingerprints of human-caused climate change which most people aren't aware of.
Most of these observed fingerprints are predicted and explained by climate models which use the known physics of the Earth's climate system and predict how it will change as the amount of greenhouse gases in the atmosphere increases. The most straightforward predictions pertain to temperature changes.
Surface Temperature Change
Back in 1988, NASA's James Hansen made the first projections of future global warming with a climate model. He created 3 scenarios which he called Scenarios A, B, and C which used various possible future greenhouse gas emissions levels. Scenario A used a model with accelerating greenhouse gas emissions, Scenario B had linearly increasing emissions, and Scenario C had emissions leveling off after the year 2000. None of these models ended up matching greenhouse gas emissions exactly right, but the radiative forcing (energy imbalance) in Scenario B was closest, too high by about 10% as of 2009. Additionally, the climate sensitivity in Hansen's 1988 model (predicting 4.2°C global warming for a doubling of atmospheric CO2) was a bit higher than today's best estimate (3°C warming for CO2 doubling).
Hansen's Scenario B projected a global warming trend from 1984-2009 of 0.26°C per decade. The actual trend as measured by surface temperature stations over that period was about 0.2°C per decade. When corrected for the 10% smaller radiative forcing than Scenario B and the higher climate sensitivity in Hansen's models, his study projected the global warming over the ensuing 25 years almost perfectly.
Meehl et al. (2004) took a different approach. Instead of projecting future surface temperature change, they used climate models to attempt to attribute past temperature changes in a method known as 'hindcasting' (as opposed to forecasting). In their study,
The late-twentieth-century warming can only be reproduced in the model with anthropogenic forcing (mainly GHGs), while the early twentieth-century warming is mainly caused by natural forcing in the model (mainly solar).
Stott et al. (2003) took yet another approach, examining surface temperature changes region-by-region across the planet and comparing them to how climate models predicted they should have changed. Stott found that regional temperature changes could also be traced back to anthropogenic global warming.
The causes of twentieth century temperature change in six separate land areas of the Earth have been determined by carrying out a series of optimal detection analyses. The warming effects of increasing greenhouse gas concentrations have been detected in all the regions examined, including North America and Europe….Our results show significant anthropogenic warming trends in all the continental regions analyzed. In all these regions, greenhouse gases are estimated to have caused generally increasing warming as the century progressed, balanced to a greater or lesser degree, depending on the region, by cooling from sulfate aerosols in the middle of the century.
More warming at night than day
Climate models predict that as a consequence of anthropogenic global warming, the planet should warm more at night than during the day. This is also known as a decreasing diurnal temperature range (DTR – the difference between minimum and maximum daily temperature). Braganza et al. (2004) investigated the changes in DTR over the past 50 years and conclude as follows.
Observed DTR over land shows a large negative trend of ~0.4°C over the last 50 years that is very unlikely to have occurred due to internal variability. This trend is due to larger increases in minimum temperatures (~0.9°C) than maximum temperatures (~0.6°C) over the same period. Analysis of trends in DTR over the last century from five coupled climate models shows that simulated trends in DTR due to anthropogenic forcing are much smaller than observed. This difference is attributable to larger than observed changes in maximum temperatures in four of the five models analysed here, a result consistent with previous modelling studies.
Essentially Braganza et al. find that that while DTR is decreasing as expected by climate models, it’s decreasing more than they predict because daytime temperatures are increasing less than they predict, possibly because the models omit changes in the Earth’s reflectivity from factors like cloudcover and land use change. Here you can see the observed changes in maximum, minimum, mean global temperature, and DTR vs. predictions by the four climate models used in the study.
Stratospheric Temperature Change
As the lower atmosphere warms due to an enhanced greenhouse effect, the upper atmosphere is expected to cool as a consequence. The simple way to think about this is that greenhouse gases are trapping heat in the lower atmosphere. Since less heat is released into the upper atmosphere (starting with the stratosphere), it cools.
Jones et al. (2003) investigate the changes in temperature over the past 4 decades at both the near surface (troposphere) and stratosphere layers, and compare them to changes predicted by a coupled atmosphere/ocean general circulation model, HadCM3. They find as follows.
Our results strengthen the case for an anthropogenic influence on climate. Unlike previous studies we attribute observed decadal-mean temperature changes both to anthropogenic emissions, and changes in stratospheric volcanic aerosols. The temperature response to change in solar irradiance is also detected but with a lower confidence than the other forcings.
The tropopause is the atmospheric boundary between the troposphere and the stratosphere. Observations indicate that the tropopause height has increased several hundred meters over the past 3 decades. Santer et al. (2003) investigate the causes of this change and conclude as follows.
Comparable increases are evident in climate model experiments. The latter show that human-induced changes in ozone and well-mixed greenhouse gases account for ~80% of the simulated rise in tropopause height over 1979–1999. Their primary contributions are through cooling of the stratosphere (caused by ozone) and warming of the troposphere (caused by well-mixed greenhouse gases). A model predicted fingerprint of tropopause height changes is statistically detectable in two different observational (“reanalysis”) data sets. This positive detection result allows us to attribute overall tropopause height changes to a combination of anthropogenic and natural external forcings, with the anthropogenic component predominating.
Upper Atmosphere Temperature Change
The layers above the stratosphere are expected to cool as a result of global warming as well, for similar reasons (less heat reaching higher levels as it’s trapped in the lower atmosphere). Jarvis et al. (1998) investigated changes in the thermosphere and ionosphere in 1998 and concluded as follows.
The estimated long-term decrease in altitude is of a similar order of magnitude to that which has been predicted to result in the thermosphere from anthropogenic change related to greenhouse gases.
Laštovička et al. (2006) arrived at a similar conclusion.
The upper atmosphere is generally cooling and contracting, and related changes in chemical composition are affecting the ionosphere. The dominant driver of these trends is increasing greenhouse forcing, although there may be contributions from anthropogenic changes of the ozone layer and long-term increase of geomagnetic activity throughout the 20th century. Thus, the anthropogenic emissions of greenhouse gases influence the atmosphere at nearly all altitudes between ground and space, affecting not only life on the surface but also the space-based technological systems on which we increasingly rely.
Ocean Heat Content
Ocean heat content has increased significantly over the past 40 years. In fact, approximately 84% of the total heating of the Earth system over that period has gone into warming the oceans. Barnett et al. (2005) investigate the cause of this warming signal, and conclude as follows.
it cannot be explained by natural internal climate variability or solar and volcanic forcing, but is well simulated by two anthropogenically forced climate models. We conclude that it is of human origin, a conclusion robust to observational sampling and model differences. Changes in advection combine with surface forcing to give the overall warming pattern. The implications of this study suggest that society needs to seriously consider model predictions of future climate change.
Sea Level Pressure
Gillett et al. (2003) compare observed changes in sea level pressure with those predicted by four coupled ocean–atmosphere climate models and find as follows.
Here we detect an influence of anthropogenic greenhouse gases and sulphate aerosols in observations of winter sea-level pressure (December to February), using combined simulations from four climate models. We find increases in sea-level pressure over the subtropical North Atlantic Ocean, southern Europe and North Africa, and decreases in the polar regions and the North Pacific Ocean, in response to human influence….Overall, we find that anthropogenic greenhouse gases and sulphate aerosols have had a detectable influence on sea-level pressure over the second half of the twentieth century: this represents evidence of human influence on climate independent of measurements of temperature change.
Zhang et al. (2007) show that models using natural + anthropogenic forcings do a much better job of matching observed precipitation trends than either natural or anthropogenic alone. The correlation with natural forcings alone is extremely weak - only 0.02. With anthropogenic alone is 0.69, and with both combined is 0.83 over the past 75 years.
We show that anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands, and that these changes cannot be explained by internal climate variability or natural forcing. We estimate that anthropogenic forcing contributed significantly to observed increases in precipitation in the Northern Hemisphere mid-latitudes, drying in the Northern Hemisphere subtropics and tropics, and moistening in the Southern Hemisphere subtropics and deep tropics. The observed changes, which are larger than estimated from model simulations, may have already had significant effects on ecosystems, agriculture and human health in regions that are sensitive to changes in precipitation
Anthropogenic global warming is caused by an increase in the amount of downward longwave infrared radiation coming from greenhouse gases in the atmosphere. Philipona et al. (2004) measured the the changes and trends of radiative fluxes at the surface and their relation to greenhouse gas increases and temperature and humidity changes measured from 1995 to 2002 at eight stations of the Alpine Surface Radiation Budget (ASRB) network. They concluded as follows.
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.
Evans et al. (2006) took it a step further, performing an analysis of high resolution spectral data allowed them to quantitatively attribute the increase in downward radiation to each of several greenhouse gases. This study went as far as to conclude
This experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming.
Trenberth et al. (2009) used satellite data to measure the Earth's energy balance at the top of the atmosphere (TOA) and found that the net imbalance was 0.9 Watts per square meter. (Wm-2) This figure is consistent with the calculations in Hansen et al. 2005 using ocean heat data.
the predicted energy imbalance due to increasing greenhouse gases has grown to 0.85 ± 0.15 W/m2
Murphy et al. (2009) obtain a similar result.
About 20% of the integrated positive forcing by greenhouse gases and solar radiation since 1950 has been radiated to space. Only about 10% of the positive forcing (about 1/3 of the net forcing) has gone into heating the Earth, almost all into the oceans. About 20% of the positive forcing has been balanced by volcanic aerosols, and the remaining 50% is mainly attributable to tropospheric aerosols. After accounting for the measured terms, the residual forcing between 1970 and 2000 due to direct and indirect forcing by aerosols as well as semidirect forcing from greenhouse gases and any unknown mechanism can be estimated as 1.1 ± 0.4 Wm-2
With such a wide variety of global and regional climate change observations strongly matching the changes predicted by climate models and providing clear fingerprints of human-caused climate change, it's no wonder that over 97% of climate scientists agree that humans are causing global warming.