What is the Evidence for Global Warming?
Carbon dioxide concentration in the atmosphere measured by David Keeling
and colleagues at Mauna Loa, Hawai'i and from polar ice cores, with
average global surface temperature of earth.
Image from Woods
Hole Research Center, presentation by Director John P. Holdren, The
The plot above, which we saw in the last chapter, shows that earth surface
is warming. Now let's look at the evidence used to make the plot.
- Where do we get our information?
- How do we know if the ocean or land temperatures are changing?
- What is the evidence?
- How good is the evidence?
Where do we get our information?
- On land, temperature is measured a hundreds of weather stations,
somewhat unevenly distributed around the world, and on some oceanic
Map of land stations in the Global Historical Climatology Network where
air temperature was measured on land and islands. From: NOAA
National Climate Data Center.
- At sea, we get data from satellites and from ships. Satellite measurements
of surface temperature come primarily from the Advanced
Very High Resolution Radiometer (AVHRR) first launched in 1978
and operated continuously since then. The satellite data are calibrated
using ship observations of surface temperature from the same time and
place. Accuracy of the combined ship and satellite data set, the Reynolds
Optimum Interpolation Sea-Surface Temperature maps is about +-
0.3 degrees C on a one-degree (horizontal) grid.
- Data from the AVHRR are available with horizontal resolution of about
1 km. Such maps show much more detail than the Reynolds maps. For example,
look at a map of sea-surface
temperature in the Gulf of Mexico produced by the Johns Hopkins
University Applied Physics Laboratory, Ocean
Remote Sensing Group. Click on a few of the thumbnails to bring
up the image.
- How was the map made?
- What problems might we have if we tried to determine average
temperature of the ocean before satellites were available, by using
data from ships?
- To learn more, look at the sample
images of the Gulf Stream.
Before 1978, all observations at sea were made from
ships using thermometers to measure water samples collected in buckets
(bucket temperature) or to measure water drawn into the ship to cool
the engines (injection temperature). Approximately 185,000,000 observations
have been collected, evaluated, and tabulated through the International
Comprehensive Ocean Atmosphere Data Set
(ICOADS) for the period
1784 to 2002. The data set is the monthly summaries of the observations.
The monthly time series are available at 2-degree (1800-2002) and
1-degree (1960-2002) spatial resolutions. Very few observations are
available before about 1850, and most are from 1900.
Number of reports of marine weather
reports each year included in the International Comprehensive
Ocean-Atmosphere Data Set
Climate Diagnostics Center
Number of reports of marine weather reports
each year included in the International Comprehensive Ocean-Atmosphere
Data Set in the period 1936 to 2005 in release 2.3 of the data
set. Click on the image for a zoom.
From International ICOADS.
For more information on measurements of water and air temperature at
sea read the page on measurements
of sea surface temperature in Climate
Change 2001: The Scientific Basis by the Intergovernmental
Panel on Climate Change.
Sources of error.
Several sources contribute errors to the plot of earth's surface temperature
- One important error is due to the large variability in the the land
and ocean temperature from region to region and month to month. Temperatures
on land vary up to approximately 15-20 degrees C during the day at
mid latitudes, and by up to approximately 50 degrees C from summer
to winter. Over the oceans, the range is much smaller, approximately
7 degrees C from summer to winter.
- The biggest error in the calculation is called the sampling error.
We do not have enough measurements to determine if temperature is changing
before about 1850, and we barely have enough even today. The error
leads to some the year-to-year variability in the plot of global averaged
surface temperature as as a function of time. Also read about the sampling
error in oceanography (scroll down to find the box on sampling
- Smith and Reynolds report that the 95% confidence uncertainty for
the near-global average is 0.48C or more in the nineteenth century,
near 0.28C for the first half of the twentieth century, and 0.18C or
less after 1950.
Global average of sea-surface temperature
calculated using Smith
and Reynolds techniques, with edtimates of errors in the values.
From NOAA National Climate Data Center Climate
2005 Annual Report.
- Instruments have some error. For example, water in buckets made of
canvas used from 1900 to 1940 cooled off quickly compared with water
in wooden buckets used before 1900. This introduced systematic, small
errors into global averages of sea-surface temperature. See Box 2.2:
Adjustments and Corrections to Marine Observations in measurements
of sea surface temperature and ocean air temperature in Climate
- The urban heat island effect. Most measurements on land are made
near cities. As cities grow, they heat the atmosphere over and near
the city. This heating is due to the city, not to global warming. About
50% of the warming in the US may be due to heat islands and land use
changes (Kalnay, 2003).
Evidence from the past 400,000 years.
The instrumental record based on direct measurements of
temperature made by thermometers and satellite instruments goes back
only a hundred and fifty years. To learn about more about earlier climate
change we need to use proxy
data, measurements of phenomena that depend on climate. Various types
of proxy data are used:
- Cores of the sea floor made by the Integrated
Ocean Drilling Program IODP. For example, Expeditions
303 and 306 collected data on climate variability in the North
Atlantic over tha past few million years. The data is used with data
cores from the Greenland Ice Sheet.
Location of proposed drill sites. Blue
circles = primary sites planned for Expedition 303, red circles
= primary sites planned for Expedition 306, and open circles =
alternate sites. From Expeditions
303 and 306 Scientific Prospectus, Introduction.
- Ice cores from
thick ice sheets in Greenland,
and mountain glaciers from around
the world provide many different types of data:
Location of Greenland ice cores. From North
Greenland Ice Core Project.
Photo of an ice core. From Kennedy
- The layers give the age of the ice. For the latest ten thousand
years of longer, counting the layers gives age.
Here is a photo of ice in a core
collected by from the North Greenland Ice Core Project
showing annual layers of the ice from about 1800 m depth,
which means the ice is about 20 000 years old.The curve
shows the variations in light intensity measured by a line
scanner showing the light intensity scattered from the
ice. This picture will be a part of Søren Wedel
Nielsen's Master's Thesis. From North
Greenland Ice core Project, photos
isotopic composition, especially the ratio (18O/16O)
where 18O is the concentration of the oxygen 18
isotope, and 16O is the concentration of oxygen
16 isotope, and the concentration of duterium. The oxygen
isotope ratio and the duterium concentration give the temperature
at which H2O condensed as water or snow on the
surface of the ice sheet.
- Air bubbles trapped in the ice gives atmospheric gas content,
especially the concentration of carbon dioxide.
- Dust content in the ice depends on windiness over land upwind
of the ice sheet.
- Salt content in the ice depends on windiness over the ocean
upwind of the ice sheet.
- Sulphuric acid content of the ice depends on volcanic activity.
- Learn more about evidence collected from ice cores by reading Deciphering
Mysteries of Past Climate From Antarctic Ice Cores.
- Finally, look at the graph of climate change over the past 400,000
years from the Vostock Ice Core from Introduction
to Climate Change to see how data from one ice core was
used to reconstruct the climate in Antarctica.
- Dendrochronology uses
measurements of the width of tree rings to determine relative changes
in environmental conditions influencing the growth of trees. Change
sin width provide information on droughts and temperature changes.
See also dendrochronology at
the Minnesota State University's E-Museum.
of pollen deposited in layered sediments in lakes gives the type
of plants growing in the vicinity of the lake at different times.
Types of plants depends on climate, and their types and abundance
give information about past climates.
- Look at the data and how data are collected at NOAA's
Paleoclimate web site, beginning with the Instrumental
Record for the past 100 Years.
- Then read the Paleoclimate
Data for the Last 1000 Years.
- Then read about Paleoclimate
Data for Before 1000 Years Ago.
Here are a few more plots: Global-average
Surface Temperature, Average
US Temperature, Many
Maps of Temperature Trends. See also Temperature
Trends at Selected Stations.
Kalnay, E. and M. Cai (2003). Impact of urbanization
and land-use change on climate. Nature 423: 528–531.
Kennedy, D. and B. Hanson (2006). "Ice and History." Science 311
a good overview of the ways to reconstruct paleoclimate.
Stokstad, E. (2001). PALEOCLIMATE: Myriad Ways to Reconstruct Past Climate.
Science 292 (5517): 658–659.
2 February, 2009