Climate models provide physics-based estimates of future warming given different assumptions about future emissions, greenhouse gas concentrations and other climate-influencing factors.
Model estimates of temperatures prior to 2005 are a “hindcast” using known past climate influences, while temperatures projected after 2005 are a “forecast” based on an estimate of how things might change.
The figure below shows the range of individual models forecasts featured in the Intergovernmental Panel on Climate Change (IPCC’s) Fifth Assessment Report – known as CMIP5 models – between 1970 and 2020 with grey shading and the average projection across all the models shown in black. Individual observational temperature records are represented by coloured lines.
Twelve-month running average of global mean surface temperatures from CMIP5 models and observations between 1970 and 2020. Models use RCP4.5 forcings after 2005. They include sea surface temperatures over oceans and surface air temperatures over land to match what is measured by observations. Anomalies plotted with respect to a 1981-2010 baseline. Chart by Carbon Brief using Highcharts.
While global temperatures were running a bit below warming projected by climate models between 2005 and 2014, the last few years have been pretty close to the model average. This is particularly true for globally complete temperature records like NASA, Cowtan and Way, Berkeley and the Copernicus reanalysis.
Sixth warmest in the troposphere
In addition to surface measurements over the world’s land and oceans, satellite microwave sounding units have been providing estimates of global lower atmospheric temperatures – the lower troposphere – since 1979.
The records produced by Remote Sensing Systems (RSS) and the University of Alabama, Huntsville (UAH) show 2018 as the sixth warmest year on record. The two records are shown in the figure below – RSS in red and UAH in blue. The surface record from NASA GISTemp is also shown for reference.
Global average lower troposphere temperatures from RSS version 4 (red) and UAH version 6 (blue) relative to a 1981-2010 baseline. The surface temperature record from NASA GISTemp is shown for reference by a dotted black line. Chart by Carbon Brief using Highcharts.
These satellites measure the temperature of the lower troposphere and capture average temperature changes around 5km above the surface. This region tends to be influenced more strongly by El Niño and La Niña events than the surface and satellite records show correspondingly larger warming or cooling spikes during these events. This is why, for example, 1998 shows up as one of the warmest years in satellites, but not in surface records.
The two lower tropospheric temperature records – UAH and RSS – show large differences after the early 2000s. RSS shows an overall rate of warming quite similar to surface temperature records, while UAH shows considerably slower warming in recent years than has been observed on the surface. Both are subject to some large uncertainties and have seen large adjustments in recent years that have warmed RSS and cooled UAH compared to prior versions of each record.
https://www.carbonbrief.org/state-of...ld-warmed-2018