Global Warming - It's All Relative Part I

Evidence that the global average temperature has increased over the last century is overwhelming.  Measurements from various sources, including ground stations, satellites, weather balloons, and from the sea, while varying in time dependence and magnitude of the temperature increase, all demonstrate clearly that the Earth is warming.  This warming is believed by many climate scientists to be the cause of some currently occurring changes in climate, and is expected to be the cause of potentially greater climate changes in the future if the increase continues. 

A critical question is to what degree is mankind responsible for the warming since the beginning of the Industrial Age, when we started to unintentionally modify the atmosphere?  The most recent complete report (AR5) by the Intergovernmental Panel on Climate Change (IPCC) asserted that is “extremely likely” that human activities are the dominant cause of the warming of the planet since the mid-twentieth century.  What is the evidence to support this claim? 

One way to evaluate this claim is to compare recent temperature and temperature increases with conditions that have occurred naturally in earlier periods of the Earth's history.  If the temperature or warming rate over the last 50 to 150 years is unprecedented that would provide key evidence that mankind is responsible for the majority of recent warming.

But defining and measuring the global average temperature is not as easy as it might seem. Figure 1 below shows one of the most commonly accepted Global Surface Temperature (GST) histories from NASA's Goddard Institute for Space Studies (GISS).  It is the historical global temperature record created from a network of ground stations and measurements at sea using thermometers. (Note that the temperature is displayed as an aberration from a mean reference temperature as is typically done when discussing climate change.)

Fig. 1 By NASA Goddard Institute for Space Studies - https://data.giss.nasa.gov/gistemp/graphs_v4/, Public

Domain, https://commons.wikimedia.org/w/index.php?curid=24363898

When newspaper & website articles or segments on the evening news report that the latest year is one of the hottest on record, they're frequently referring to data from GISS.

But, there are critics of this claimed historical temperature record. One common complaint is that this temperature record has been manipulated to try to accentuate the recent increase.  This complaint is not without merit. Two of the principals at GISS, first James Hansen and then Gavin Schmidt, have been among the loudest proponents of drastic climate change action.  Critics claim that the numerous adjustments to this temperature record always accentuate recent increases to bolster the alarmist position. The GISS website acknowledges the modifications to the historical record with the graph below in Fig. 2 which shows GISS historical temperature analysis at several different years in the past compared to the present analysis. 

Fig. 2. Graphical display of GISS temperature history adjustments

While it is true that the GISS adjustments to the temperature data have accentuated recent temperature increases, it's also true that temperature is clearly rising in the GISS data no matter how the data has been analyzed.

A more valid criticism of this temperature analysis is that the temperature record from land based stations has been polluted by the urban heat island (UHI) effect.  Almost everyone who lives near a large urban center is aware that the temperature in the urban core can typically be many degrees higher than surrounding areas. Over the last century, as towns having temperature stations grew into cities, the average of recorded temperatures would rise along with the degree of urbanization. This temperature increase introduces a false positive trend to GST over time which is difficult to address via adjustments.  In addition to the UHI effect, the thermometer-based temperature record has other issues such as some of stations being moved from one place to another and changes in the technology and measurement methodology over the last century. 

To quantify the urban heat island effect and other possible inaccuracies let’s compare the GISS temperature history with the temperature history measured by satellites, which provide the most accurate and comprehensive historical global temperature record since 1979.  Fig. 3 below shows a plot of the global temperature anomaly in the lower troposphere (LT) from satellites provided by the University of Alabama Huntsville (UAH).

Fig. 3.  Temperature history from satellite measurements

This temperature analysis is consistent with temperature analysis done by other groups

studying satellite data (RSS), as well as, temperatures recorded from weather balloons.  The lower troposphere in this analysis corresponds to the atmosphere between the surface and approximately three miles from the surface.  

Both the GISS and UAH temperature analysis from 1979 to 2020 show a temperature increase that is close to linear, with fluctuations superimposed upon the linear trend.  The big difference is that the GISS temperature trend is increasing at a rate 43% higher than the data from satellites. It's possible that a portion of this large difference in the temperature increases over the last 40 years reflects a real temperature difference between the earth’s surface and the lower troposphere, but as neither theory or models predict such a large difference, it is more likely due to the limitations and issues in the data and analysis used to create Fig. 1.  For all subsequent posts and analysis we will assume that the global temperature is increasing approximately 0.14 degrees C/decade as derived from the more reliable satellite data. 

The next blog Global Warming - It’s All Relative Part II will investigate how the current temperature and rate of change compare with earlier periods of Earth’s history.