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© Eugene S. Takle
2000, 2002, 2006
Introduction
Measurements of meteorological variables with sufficient spatial coverage to represent
global observations began in the mid to late 1800s. Measurements are taken (1) a few feet (~1.6 m)
above the earth's surface over land, (2) at various elevations above the surface over land by
launches of weather balloons every 12 hours, and (3) a few feet above the water surface over oceans.
On-going measurements also are taken by satellites, but this topic is covered in
Unit 2-11 . It is important to
remember that, in our discussions about global warming, the term "surface temperatures" really means
air temperatures taken a few feet above the surface of the earth. These are the temperatures
that are normally associated with weather forecasts reported by the
media.
Measurements Over Land
Measurements made
at land stations include temperature, dew point, relative humidity, precipitation, snowfall,
snow depth on ground, wind speed, wind direction, cloudiness, visibility, atmospheric pressure,
evaporation, soil temperatures, and various types of weather occurrences such as hail, fog,
thunder, glaze, etc. In the US, weather observations are normally taken at hourly intervals, but
sometimes are taken more frequently under rapidly changing conditions. New and automated weather
stations report a more limited list of meteorological variables at 20 minute intervals.
Weather data from the atmosphere above the earth's surface (above about 3 meters) are taken by instrument packages (radiosondes, rawinsondes, and rocketsondes) that are carried aloft by weather balloons that are launched every 12 hours (in the US and many other locations, but less frequently in developing countries) and transmit data to a receiving station on the ground. These "upper air" data include temperature, relative humidity, atmospheric pressure, and wind. Vertical profiles of atmospheric measurements taken at approximately the same time are called atmospheric soundings.
Measurements Over Oceans
Measurements over oceans are
taken at the locations of buoys that may be either moored or floating and from ships at sea.
These observations normally are reported at hourly intervals. A global ocean observing system,
GOOS, is in the planning stages that envisions "a global network to systematically acquire,
integrate and distribute marine and oceanic observations, and to routinely produce analyses and
forecasts and related products to enable governments, industry, science and the general public
to cope with critical marine-related issues including the ocean's effects on climate."
Global Mean Surface Temperature
Research teams from the US, Great Britain, and Russia have
independently analyzed global records of air temperature taken at surface.
These data sets begin about 1860 and
extend to the present.
Studies of global warming that rely on surface air temperature measurements generally use plots of global (or hemispheric) mean temperature that are expressed relative to some base temperature (Figure 1). Typically, the 1951-1980 average or the 1961-1990 average is used as a reference, and individual annual temperatures are expressed as their departure from this mean value.
Problems Leading to Uncertainty
There are several problems that lead to uncertainty in constructing these
hemispheric and global mean temperature records (Houghton et al, 1990):
Houghton (1990) has considerable discussion on each of these items except 6, which is a more recently described factor. They conclude that item 5, and urbanization in particular, is the largest source of possible error in the temperature record. As cities have expanded, formerly rural weather stations become surrounded by, or at least are influenced by, urban landscapes, which are known to have higher surface air temperatures. This source of error differs from the others primarily because, whereas the others may have some stations giving positive errors while others may give compensating negative errors, all stations subjected to urbanization will cause a monotonic warming of the record. However, analysis of temperature records from stations known to be continuously located in rural areas show the same general pattern as the complete data set of all stations.
Temperature Trends
Oceans account for 61% of the surface area in the Northern Hemisphere and 81% of the area in the
Southern Hemisphere. Over the ocean the temperature patterns generally resemble the land-based
record (these are discussed further in Unit 2-9).
The combined temperature record, Figure 1, shows a relatively stable period from 1860 - 1910 followed by a rise to about 1940. After this time, the record has relatively weak downward trend but with higher variability until about 1975 when the rather abrupt rise to the present value begins.
If we look separately at daily maximum and daily minimum temperatures, Figure 2, we find that a rise in daily minimum temperatures is accounting for most of the warming that has been observed, at least in the US and certain parts of the world. This could be attributed to both rise in greenhouse gases and increases in cloudiness, which will have a larger influence on nighttime minimum temperatures than daytime maximum values.
Precipitation
Analysis of precipitation records is even more difficult than temperature. Errors in precipitation
records include wind speed during rain/snow events that causes droplets/snowflakes to be deflected from
falling in the gauge by flow patterns around the instrument. This error always leads to reports of
rain being less than actually occurred, so it (like urban heat islands were for temperature) does not
experience a compensation effect when a large number of stations are aggregated. Elaborate means can be
taken to compensate for this, but such accurate instruments require more expense and maintenance.
Wetting of the gauge in light precipitation events also can lead to under-reporting of
precipitation amounts. Even if the gauges were accurate, the spatial variability of rainfall
is much larger than that for temperature, so intense rain events can be missed completely by a
sparse network. Also, intense but isolated rain even at one station in a sparse network can be
erroneously inferred to be representative of rainfall over a large area. Because this
representativeness problem may cause either positive or negative errors, a large measure of
compensation usually occurs.
Analysis of precipitation records, Figure 3, over the globe show some regions seemingly experiencing a trend toward increasing precipitation and others decreasing. Subtle changes in large-scale flow patterns can influence precipitation patterns, so it is difficult to draw general conclusions. A global distribution of change from 1900-1994 generally shows increases at high latitudes and decreases in subtropical areas. (Figure 4)
Temperature Above the Surface
Temperatures in the atmosphere above the surface,
Figure 5, as measured by balloon-borne instruments and satellites have been analyzed
for trends over the period of record which, for these observations, is only about 50 years long.
The middle troposphere from about 1,500 m to 7,000 m (850 mb - 300 mb) shows a general warming
since about 1965 of about 0.4°C. The upper troposphere (300mb - 100 mb) shows a cooling
trend in general disagreement with most global climate models. The lower stratosphere generally
shows a substantial cooling, which is in agreement with most model results and is due to
reduced ozone absorption of UV radiation and increased warming at the surface which deepens
convection.
Ocean Water Temperatures
There are some measurements,
Figure 6, taken from the North Atlantic and North Pacific Oceans that suggest a
warming of the ocean layer from 500 m to 3000 m depth. Areal coverage by
snow, Figure 6, in the Northern Hemisphere
shows a gradual decline over the last 20 years during which satellite measurements have been made.
Sea ice area, Figure 7,
over the same period shows a slight decline in the Southern Hemisphere and essentially
no change in the Northern Hemisphere. Perhaps a more meaningful measure of change in sea ice
volume, however, is sea-ice thickness rather than horizontal areal
coverage. There have been
reports of substantial thinning of ice in recent years,
although the record is quite short (Figure 8).
