What is “global temperature”?
How do you measure it? What are the biases?
We covered several important figures from the IPCC report including global mean temperature and its abrupt increase in recent years. When first published, such temperature changes associated with atmospheric CO2 concentration increases were eye-opening for most of us and raised strong awareness that something had to be done about greenhouse gas emissions. At the same time, related scientific findings were repeatedly targeted and attacked by politicians and lobbyists.
It is important for you to understand the “global mean temperature”, so that, in the future, if and when you encounter the same question, you will have an answer with a firm scientific foundation.
Let’s think about how we measure “global” temperature. Think about the temperature variations you experience daily. How can we know what the ”global” temperature is when we don’t even know exactly what our backyard temperature is? Furthermore, different parts of Earth have varying temperatures: tropical versus arctic, day time versus night time, bottom of a valley versus top of a mountain, under your backyard tree versus sunny outdoor porch, etc. Are these varyingtemperatures included in the “global” temperature? Even cities at same latitude do not have same mean annual temperature. For instance, cites situated around 42N, which include Springfield in MA, Chicago in IL, Norfolk in NE, Providence in RI. Do these cities have same mean annual temperature?
No. Cities near the coast show a milder maritime climate, while intercontinental cities such as Norfolk in Nebraska show greater temperature differences in day and night/summer and winter.
Since there is great variety in temperatures, how do we obtain global temperature?
Please visit interactive website to plot global temperature anomalies from 1880 to 2020https://www.ncdc.noaa.gov/cag/global/time-series/globe/land_ocean/ytd/12/1880-2020
How do you measure “global” temperature? How about CO2?Is the world really warming?
Global mean temperature is the mean of direct temperature measured from all over the world, including both land and sea. In other words, it is literary a mean of as many possible measurements for as long as possible. That being said, the number of direct measurements has changed significantly throughout time. We had only a few temperature measurement stations in 1850 while we now have thousands of measurements. With this, looking to the past, there was a greatererror margin in the earlier years (shaded colored area in the following figure).
As you have learned through previous lectures, especially during the glacial -interglacial cycles, temperature change, measured through air bubbles preserved in Greenland and Antarctic ice cores, coincide with CO2 concentration, as well as other greenhouse gas concentrations. In recent years, direct measurement of CO2concentration in the air has steadily increased since 1958 (Keeling Curve), which coincides with recent increases in global mean temperatures.(NOAA National Centers for Environmental information, Climate at a Glance: Global Time Series, published March 2021, retrieved on April 8, 2021 from https://www.ncdc.noaa.gov/cag/)
Further reading: Global Temperature Anomalies from 1880 to 2020, January 14, 2021https://svs.gsfc.nasa.gov/4882New Studies Increase Confidence in NASA's Measure of Earth's Temperature, May 23, 2019https://climate.nasa.gov/news/2876/new-studies-increase-confidence-in-nasas-measure-of-earths-temperature/
Global mean temperature needs to weight on some biases associated with the
temperature measurements. Specifically, as you see in this figure, land surface
temperature and sea surface temperature show different rates of increasing. Two
important biases to learn are associated with differences in land and ocean. First,
urban areas tend to heat up faster than rural areas that retain more moisture in
bare soil than in developed urban areas. Also, the ocean, as a body of water, has a
much greater heat capacity than the land surface, and can replace surface water
with colder deeper water (therefore, the ocean tends to be cooler than land
surface). In order to overcome such biases, scientists from different organizations
(Meteorological office Hadley Center and Climate Research Unit in UK, NOAA
National Centers for Environmental Information, and NASA Goddard Institute for
Space Studies) independently create global mean temperatures to verify that
they are reliable. All data and software used for the reconstruction of global
mean temperatures is available to the public.
Here is a relevant, useful and interesting web articles:
Further ReadingAre surface temperature records reliable?
https://skepticalscience.com/surface-temperature-measurements-advanced.htm
In Merchants of Doubts, Dr. Fred Singer stated: "In the last ten years, there
hasn't been a warming. We don't know why that is. But one doesn't see any
warming in the observations. There simply is no trend.”
Is this really true? Let’s observe the data and comprehend what is going on.
The year of 2020 was reported to be the warmest year on record tied with
2016. Not only that, since the past 6 years were in the range of the warmest
years on record, scientist are now thinking that this increasing trend will
become the new norm.
Global temperature anomalies 2009-2013 (look at the north pole!)
Polar amplification is an important factor for cryosphere feedback. It is caused by additional poleward energy transport, high-latitude water vapor content, and cloud feedbacks.
Also, it is important to remember polar amplification, which you learned in previous lectures. Global temperature change does not occur evenly. Rather, it shows preferential hot spots. the arctic region is one of them. Important factors that cause such preferential warming in the polar region is cryosphere climate feedback (Snow/ice-albedo feedback), cloud feedback, additional poleward energy transport through wind and ocean, and the change in the amount of water vapor at high-latitude.
Fig 5, IPCC Scientific Basis, WG1, Technical Summary: (Mann et al., GRL, 1999).
Northern Hemisphere Temperature record: Last 1000 years
This figure, originally published in 1999, is called the “Hockey Stick Curve” and has been
cited over 2300 times in peer reviewed journals by climate scientists and policy makers
(which is a significant number!). Particularly, the figure has made a broad audience
aware of acute changes in the Northern Hemisphere average temperature over the past
100 years. As climate studies are seeing greater importance, this paper has been a
target of dispute by climate skeptics specifically tied to the energy industry (e.g.
McIntyre and McKitrick 2003).
Original publication: Mann et al. 1999, Northern Hemisphere temperature during the past millennium: inferences, uncertainties, and limitationshttp://www.meteo.psu.edu/holocene/public_html/Mann/research/res_pages/ONLINE-
PREPRINTS/Millennium/mbh99.pdf
McIntyre and McKitrick 2003, Corrections to the Mann et. al. (1998) Proxy data base
and Northern Hemispheric average temperature series
https://journals.sagepub.com/doi/10.1260/095830503322793632
