A new research paper by Lawrence Livermore National Laboratory climate scientist Ben Santer and co-authors looks in detail at how climate change resulting from human activities is affecting the temperature of Earth’s atmosphere.
They argue in their paper in the Proceedings of the National Academy of Sciences (PNAS) that natural climate forcings like volcanoes, El Niño, and changes in solar activity could not have been responsible for the cooling of the upper atmosphere and warming of the lower atmosphere, and they identify a clear human “fingerprint” to the warming seen over the last 30 years. While observational data from satellites show less warming than predicted by most models, Santer and his co-authors demonstrate that the observed warming is consistent with models including both human and natural forcings, but inconsistent with models using only natural forcings and variability.
To determine effects of both natural climate forcings and the human contribution, the researchers examined global climate model runs from the latest set of models, known as CMIP5, produced for the IPCC Fifth Assessment Report. They compared temperatures at different layers of the troposphere — the lower part of the atmosphere to six or so miles up — and lower stratosphere (between six to 30 miles up). They looked at both the standard model runs, which include anthropogenic (human-caused) and natural forcings, and also at runs that only include natural forcings. That approach let them try to isolate the anthropogenic component of warming, allowing them to see if it was statistically significantly different from what likely would have happened in the absence of human activities.
Their research approach has been used extensively before, but applied mainly to surface and ocean temperatures. The figure below, taken from the 2007 IPCC report, shows model runs with only natural forcings; model runs with all forcings; and observations of surface temperatures for the whole globe — land areas and ocean areas. The fifth IPCC report was expected to expand these graphs by showing a similar chart for deeper ocean temperatures (0 to 2000 meters).
In all cases, the observed temperatures are generally consistent with model runs incorporating all forcings and notably inconsistent with runs using only natural forcings.
Bottom panel of Figure SPM.4 from the 2007 Intergovernmental Panel on Climate Chance (IPCC) Fourth Assessment Report (AR4) Working Group One (WG1) Summary for Policy Makers (SPM).
To determine the temperature of different portions of the atmosphere, Santer and his colleagues sampled the output of global climate models at specific areas of the atmosphere where temperature is currently measured by satellites. This approach allowed them to compare the rate and distribution of warming predicted by models with those shown in observations. The figure below provides a rough schematic to help visualize how climate models work. They calculate changes in heat flows, moisture changes, and other factors in three-dimensional grid boxes — and do so in Earth’s atmosphere, at its surface, and beneath the surface of the oceans — with grid boxes interacting with their neighbors.
Simplified schematic of a Global Climate Model from NOAA.
Santer et al provide results for the temperature of the lower stratosphere (TLS), the mid- to upper-troposphere (TMT), and the lower troposphere (TLT). The TLS and TLT tend to be of the most interest to climate researchers, as each shows different identifiable patterns of changes caused by human activities.
The lower stratosphere, approximately between six and 30 miles above Earth’s surface, is mostly above the layer of greenhouse gases that trap heat. Three main factors influence stratospheric temperatures: major volcanic events, ozone depletion, and greenhouse gas emissions. The figure below shows the lower stratospheric temperature results from climate models using both all forcings and natural forcings only from 1880 to 2012. Note that the natural forcings runs go up only to 2005.
Based on data shown in Figure 1 of Santer et al 2013
Spikes in temperature are caused by major volcanic events, which push sulfur dioxide and other aerosols into the lower stratosphere. These particles reflect some incoming solar radiation away from Earth, heating the atmosphere above them and cooling it below them. Climate models attribute most of the stratospheric cooling seen in the last 40 years to ozone depletion, with a small contribution from reduced outgoing heat via greenhouse gases. Atmospheric greenhouse gases are mostly concentrated in the upper troposphere, and increased concentrations will reduce the amount of heat radiated up into the upper atmosphere (and into space) until Earth heats up.
In contrast, tropospheric warming is driven mostly by greenhouse gases trapping additional heat in the lower part of the atmosphere. The figure below shows climate model outputs for lower tropospheric temperatures from 1880 to 2012, comparing natural-only and all-forcing runs. In the troposphere, major volcanic events have a strong cooling effect, as stratospheric aerosols reflect away some incoming solar radiation before it enters the troposphere. While the stratosphere recovers from volcanic events quite quickly, the troposphere takes longer as some heat is transferred into the oceans, where cooling-down and heating back up take time.
Based on data shown in Figure 1 of Santer et al 2013
Santer and his co-authors found that climate model outputs using all forcings and also observational satellite datasets (from UAH and RSS are significantly different from natural-only runs over the past 30 years. However, satellite observations are notably cooler in the lower troposphere than predicted by climate models, and the research team in their paper acknowledge this, remarking: “One area of concern is that on average… simulations underestimate the observed lower stratospheric cooling and overestimate tropospheric warming… These differences must be due to some combination of errors in model forcings, model response errors, residual observational inhomogeneities, and an unusual manifestation of natural internal variability in the observations.”
This can be seen clearly if changes in satellite observations in the lower troposphere since the 1980s are compared with projected changes from the all-forcings and natural forcings-only runs. While both UAH and RSS records are warmer than the natural forcings-only models, they are notably cooler than the all-forcings runs as shown in the Figure below.
Based on data shown in Figure 1 of Santer et al 2013, data from UAH, and data from RSS.
A more comprehensive comparison of models and observations is shown in Figure 2 of the Santer et al paper, reproduced below. This figure shows the trends in temperatures from 1979 to 2012 (or to 2005 in the case of the natural-forcings models shown in panels E, F, and G) by both latitude and altitude.
While complex, the take-aways are that the observations, shown in panels H and I, are much more similar to the all-forcings model runs (panel A) and the anthropogenic forcings-only runs (panel D) than to the runs using either all natural forcings (E) or only specific natural forcings (volcanoes only in F, solar only in G). However, models do not get quite the same distribution of warming seen in the observations; the observations tend to show less tropospheric warming and more stratospheric cooling in tropical regions (e.g. 20 South to 20 North).
Figure 2 of Santer et al 2013 showing temperature trends by latitude and altitude. Panels A and D show 1979 to 2012 trends in temperature from all-forcings and anthropogenic-only forcings. Panel E shows all-natural forcings, F shows volcanic-only forcings, and G shows solar-only forcings from 1979 to 2005. Panels H and I show observed trends from RSS and UAH satellites from 1979 to 2012.
Santer et al in the PNAS paper also provide some additional tests in examining scenarios with much greater solar and volcanic forcings than observed in the real world. They find that these still could not account for observed stratospheric cooling and tropospheric warming.
Computer model estimates of the ‘human influence’ fingerprint are broadly similar to the observed pattern. In sharp contrast, model simulations of internal and total natural variability cannot produce the same sustained, large-scale warming of the troposphere and cooling of the stratosphere. So in current climate models, natural causes alone are extremely unlikely to explain the observed changes in the thermal structure of the atmosphere.