ÐÓ°ÉÔ­´´

If someone asked you to find out if the Earth’s climate had changed
over the past century, your first instinct would be to reach for the meteo-rological
records – just as climate change researchers have done for decades. But
another, less obvious source of information might give you a better answer:
boreholes.

Over the past fifty years, mineral, oil and gas exploration companies
have drilled thousands of boreholes all over the world. In some cases,
geophysicists have drilled boreholes to study how temperature varies with
depth in the upper sections of the Earth’s crust. Now climatologists are
recognising that these measurements – many of which were discarded as unimportant
by the geophysicists – offer powerful clues to the Earth’s past and, possibly,
future climate.

Already, analyses of temperature readings from boreholes are producing
provocative findings. They suggest that at least part of the global warming
seen in the meteorological records of the past century can be explained
by natural fluctuations in the Earth’s temperature.

The secrets are locked away in the Earth’s crust. Geophysicists have
known for a long time that the crust becomes progressively warmer as you
bore into it, edging closer to the Earth’s hot interior. Mostly, this temperature
gradient is smooth, increasing by between 10 degree C and 50 degree C
with every kilometre from the surface. The exact amount depends on how effectively
the rock conducts heat through the crust towards the surface. But within
200 to 300 metres or so of the surface, things become less predictable.
Variations in ground temperature, caused in turn by variations in surface
air temperature, make the temperature of the underlying rock more variable.
Previously, geophysicists were interested only in measuring heat flow from
the Earth’s centre, so they threw away these unreliable top sections of
their borehole temperature data.

As climatologists now realise, however, this temperature variability
is a powerful source of information about past climatic fluctuations. In
particular, it can tell you about daily and seasonal variations in surface
temperature. These variations send what are in effect thermal ‘waves’ –
with amplitudes of 10 degree C or more – slowly down through the rock. In
temperature measurements from boreholes, these ‘waves’ show up as patterns
superimposed on the background temperature gradient.

The temperature a metre down is an accurate average of the ground temperature
the previous day. Similarly, the temperature at 20 metres is an accurate
measure of the average ground temperature over the previous annual cycle.
But the real value of the thermal waves is not in revealing yesterday’s
average ground temperature, or even last year’s. Far from it: heat travels
so slowly that the first 500 metres of crust offers a record of the Earth’s
ground temperature for the whole of the past millennium.

Records and models

One complication in exploiting this record is that heat travels at a
different rate through different types of rock. Fortunately, geologists
can calculate the rate from a constant called thermal diffusivity that
is known or measured for each rock type. For most rocks, a measurable change
in surface temperature takes a year to travel 16 metres, 100 years to travel
160 metres and 1000 years to travel 500 metres.

Underground rock temperatures are not the only record of our past climate,
nor even the most widely used. Meteorological records are what form the
basis of the complex computer models used to predict how our climate might
change as a result of the greenhouse effect, for example. These computer
models, called general circulation models, or GCMs, are complicated simulations
of the Earth’s response to changes caused by human activities such as burning
fossil fuels. They are built up from known patterns of climate change over
the past century or so. But predictions based on GCMs differ widely. One
reason for this variation is the lack of reliable meteorological data extending
beyond the past century. This is a gap borehole data might fill.

According to the meteorological record, the average air temperature
at the Earth’s surface has increased by about 0.5 degree C in the past 100
years. This warming is uneven: arctic regions show most warming, regions
at low latitudes show little or none, and some regions in Africa show slight
cooling. But making sense of these trends is difficult because reliable
and widespread records exist only for the past half century. The longest
records are usually from urban areas and have been contaminated by heat
from human activities. Many of the early weather stations were abandoned
or moved elsewhere when people did, without any corresponding adjustment
of the records. And measurement methods have varied from place to place.

Historical thermometers

Longer-term information about the climate comes from indirect sources
such as tree rings and pollen. But this is not free from error either (see
‘Hidden climate clues’). And to go further back in time, researchers must
use ice cores and sediments. While the latter will reveal large events like
ice ages, they are not detailed enough to reveal smaller climate changes.

Hence the excitement about boreholes. Edward Bullard of the University
of Cambridge made the first borehole measurements in 1939 in South Africa.
But he was interested in heat flow in the Earth, not climate. Research aimed
at investigating climate change only took off in earnest in 1986, after
researchers published the first detailed analyses of temperatures from boreholes
in Alaska and eastern Canada. Eight years on, geophysicists have measured
heat flow at 10 000 boreholes on continents worldwide. New measurements
are being added at about 200 sites per year. A global network of these historical
‘thermometers’ is fast developing.

Not all the data will be suitable for studying climate change. Boreholes
less than 150 metres deep are too shallow to extend the climate record back
beyond what is known from meteorological data. At some of the older boreholes,
researchers chose not to measure temperatures in the first 100 metres below
the surface because they thought the data would be unreliable. At others,
the rock samples were inadequate for measuring their thermal conductivity.
But an es-timated one in ten boreholes should be suitable for climate studies.
Analysing data from these sites should take between three and five years.

Re-examining existing geothermal data from boreholes has already produced
some fascinating results (see Figure 2). In 1986, for example, Arthur Lachenbruch
and Vaughan Marshall of the US Geological Survey studied a series of boreholes
spread across 500 kilo-metres of arctic Alaska. They found that the temperature
near the ground surface, the top of the permafrost, had increased by between
2 degree C and 5 degree C during this century. These variations extended
100 metres below the surface, strengthening the idea that a period of warming
began early this century.

The picture emerging from such studies is broadly in keeping with climate
patterns detected with other techniques. Recently, three research groups
at the University of Quebec at Montreal, the University of Western Ontario
and the Geological Survey of Canada looked at borehole data from 120 boreholes
at 56 sites from Manitoba to Newfoundland. They conclude that a large area
of eastern and central Canada has warmed by between 1 degree C and 2 degree
C over the past 100 years. This fits in with meteorological records and
with tree-ring data for the same region.

Testing the method

However, not all geothermal studies reveal the same pattern of warming
in Alaska and eastern Canada. In 1990, Tim Chisolm and Dave Chapman at the
University of Utah, reported on seven boreholes in the Great Basin of western
Utah. Their analysis shows little or no warming. In fact, one shows a slight
cooling. On average, surface temperature in western Utah appears to have
changed by as little as 0.3 degree C in this century. This seems to imply
that global warming may not be so global.

These Utah sites provide an ideal test of the borehole method. Unlike
their counterparts in Alaska and eastern Canada, they are dotted among a
network of weather stations that have operated for about a century. Furthermore,
these stations are in small farming communities in rural areas far from
any urban heat contamination.

Encouraged by these factors, Chapman and Chisolm, have taken the research
a step further. Using records of air tem-perature from the Utah weather
stations – which stretch back a full 100 years – the researchers have estimated
how temperature should vary with depth. The theoretical profiles agree strikingly
with the borehole data. In fact, the only weather station to predict a cooling
trend turned out to be just 50 kilometres from the one Utah borehole with
a cooling trend.

Similarly striking results are emerging elsewhere. Robert Harris, a
graduate student at the University of Utah, extended the approach adopted
in western Utah to the Colorado plateau of southeastern Utah. Last December,
he reported an average warming there of 0.6 degree C over the past 150
years. Over the past century, this trend fits with data from local weather
stations. But the data from 150 years ago seem to expose a discrepancy.
The average ground temperature calculated from the borehole analysis is
about 0.5 degree C warmer than the temperature expected from projecting
weather station records backwards.

This has important implications. While the meteorological data for this
region suggest a 1.2 degree C increase in temperature between 1890 and
1990, the borehole measurements suggest only half this, when previous centuries
are taken into account.

There are other signs that the meteorological record may overestimate
the amount of global warming that has taken place over the past century.
Take the case of the Little Ice Age. Jean-Claude Mareschal and Beltrami
have analysed borehole data from central and eastern Canada, in the process
uncovering evidence of a widespread cold period centred around 1800 where
average temperatures were depressed by 1 degree C. This coincides with
the Little Ice Age that has been documented in Europe. Did a similar climatic
fluctuation occur in eastern Canada?

The borehole data are the strongest evidence so far that it did. But
if that is so, then not all the warming subsequently detected in meteorological
records can be due to a build-up of greenhouse gases. Some of it must simply
reflect temperatures climbing back to the levels that prevailed before the
Little Ice Age. At least half of the 1 degree C of warming recorded in the
meteorological records for eastern Canada could be due to this kind of
recovery (see Figure 3).

The idea that meteorological records overestimate global warming will
be unwelcome to researchers who base their climate models on them. Could
the bore-hole measurements be deceiving us? It is certainly true that reconstructing
surface temperatures from bore-hole data is no easy task. Climate change
is not the only thing that can influence the temperature gradient through
a rock. A rock’s thermal conductivity may change with depth. Heat may be
generated in rocks as a by-product of natural radioactivity. The tilt
of the ground relative to the Sun, or changes in vegetation or snow cover,
may lead to local variations in warming.

Fortunately, it is possible to remove such effects from the final results
by combining data from several nearby boreholes. The conditions at each
hole are unlikely to be identical, so they cancel each other out. Similar
climate variations, on the other hand, add up to give a much stronger signal
than at individual sites.

A bigger drawback stems from the fact that the borehole approach can
only detect average temperatures. We say that a temperature deviation at
160 metres is linked with a climate change that happened 100 years ago,
but this temperature deviation is an average. It is an anomaly on a thermal
gradient produced by large numbers of overlapping thermal ‘waves’, all from
different years. The temperature is centred on 100 years ago, but is more
likely to be an average of the surface temperatures of between 60 and 140
years ago.

The downside of all this is that past temperature changes cannot be
pinned down to exact dates. But there is a positive side: being an average,
the temperature is accurate to within about 0.2 degree C. And it may yet
prove possible to overcome the problem of dating the temperatures to a certain
year or decade. One solution might be to combine borehole results with those
from tree rings or ice cores.

Several researchers are now pursuing this worldwide. For our part, we
are working with Yves Bergeron at the University of Quebec on a method of
combining tree-ring data with geothermal data from Canada. Already, Beltrami
and Alan Taylor of the Geological Survey of Canada have combined borehole
data with oxygen isotope data from the Canadian Arctic. Tree-ring widths
have a yearly resolution but can be affected by factors other than climate
change. Data from oxygen isotope measurements are sensitive to changes
in precipitation. But by combining these measurements with borehole data
– which suffer only from being hard to date – it is possible to build up
a complete picture. Our latest results show that the Canadian Arctic has
warmed by about 3 degree C over the past century. This is important because
the Arctic has a profound influence on climate and is where warming is most
apparent in the meteorological records. It is also the area most sensitive
to greenhouse warming.

Several groups have set up weather stations next to boreholes so that
they can continuously monitor meteorological data alongside ground and borehole
temperatures. This will enable them to study details of how air temperature
changes are recorded by rock temperatures at different depths. For the past
six months Chapman and Scott Putnam of the University of Utah have been
running an observatory at one of our borehole sites in the Great Basin in
Utah. Other thermal observatories are being operated in the Northwest Territories
of Canada, on the Great Plains of the US and in an urban setting of the
Czech Republic.

Meanwhile, Beltrami and Mareschal have developed another way of improving
the accuracy of borehole measurements: repeat the measurements at each site
at regular intervals. The question is, how often? Theory tells us that,
as it is possible to measure temperatures to within 10 millikelvins, temperature
measurements need to be repeated around every five years to pick up early
warnings of real climate change.

The next step will be a global analysis of long-term climate changes
inferred from borehole temperatures. Over the next three to five years,
Henry Pollack of the University of Michigan will lead a project that will
go some way towards this by analysing many more of the 1000 potential sites
spread around the world.

This is one of the biggest projects so far. It is being run by the International
Heat Flow Commission (a working group within the global geophysical organisation,
the International Union of Geodesy and Geophysics). Pollack and his colleagues
are already compiling all the available geothermal data from Africa, Australia,
South America and Asia that are suitable for climate analysis.

No single record is likely to provide a reliable picture of past climate
changes. The ‘final answer’ will come out of a combination of indicators
developed by experts from various disciplines. But borehole measurements
have extended the meteorological record back another two centuries and extended
geographic coverage to the critical polar regions. They look set to play
a significant role in the climate change debate.

Hugo Beltrami is a geophysicist at McGill University in Montreal and
David Chapman is a geophysicist at the University of Utah, Salt Lake City

* * *

Heat underground

Ground temperature at the Earth’s surface is a balancing act between
incoming sunlight and outgoing heat. Temperature below the Earth’s surface,
on the other hand, is affected by heat being conducted from the Earth’s
interior.

A typical amount of heat passing through each square metre each second
from this geothermal source on continents is 0.06 watts per square metre
(1 W/m² metre is the energy flux you would receive when standing
2.2 metres from a glowing 60-watt light bulb). This is small in comparison
with the solar flux of about 100 W/m², but is consistent with
an increase in temperature of between 20 degree C and 30 degree C per kilometre
of depth as the Earth’s heat is conducted to the surface.

There are standard formulas for calculating the subsurface temperature
from a surface temperature history. For example, a cooling of the Earth’s
surface by 1 degree C from 500 years ago to 200 years ago would produce
a ‘signal’ that extends to a depth of 450 metres. But it would fall furthest
below the normal (-0.21 degree C) at a depth of 150 metres. Instead of
appearing as sharp blips in the records, however, such signals tend to become
smoothed out: over time the flow of heat through rock invariably acts to
reduce the amplitude of such temperature drops.

The story would be different for an upwards or downwards trend in temperature
that began in the past and continued today. In this case, the amplitude
of the signal would be biggest at the surface and would tail off with depth.
According to the standard formulas, a change in surface temperature that
began 200 years ago would have penetrated to about 290 metres by now; while
a change that began only 10 years ago would have little or no impact on
the background gradient below 65 metres.

What if there were several changes in surface temperature, one after
another? The temperature signals from each change would become superimposed,
giving a combined trend. The beauty of boreholes is that they provide such
detailed temperature measurements that you can work backwards from the overall
pattern of signals and estimate past changes in surface temperatures
(see Figure 1).

* * *

Hidden climate clues

There are several indirect sources of climate data. One is tree rings.
Tree growth depends partly on temperature, so the width of an individual
tree ring can be used to identify how past air temperatures have varied.
A tree grows so many centimetres each year, so trees can indicate annual
changes in temperature over the past thousand years. But tree growth depends
on other things, too. Ring widths vary according to the species and age
of the tree, the amount of nutrients in the soil and the weather – the amount
of sunlight, temperature and pattern of annual rainfall.

A second indirect source is provided by two common oxygen isotopes found
in ice cores. These isotopes have a slightly different mass, and how they
distribute themselves in snow crystals in the atmosphere depends on the
air temperature. This ‘isotope signature’ of the air temperature during
snow storms is preserved as the snow is buried and compacted into ice. So
an ice core, like tree rings, records annual variations in temperature.
Polar ice in Greenland and Antarctica represents a record of atmospheric
conditions that is thousands of years old.

But temperatures estimated from oxygen isotopes in ice may not always
represent a good average temperature for that year. If the heaviest snows
fall in March, when it is warmer rather than January when temperatures are
lower, the isotope record would indicate warming even though the average
temperature for the year might be constant or might even drop. The coldest
spells might not coincide with snow fall and so might not even be recorded.

Other clues to past climate trends can come from vegetation, in the
shape of pollen and spores found in bogs or fossils. As the climate changes,
existing flora may not survive and new species that are better suited to
the new conditions slowly establish themselves. Pollen records show slow
and broad vegetation shifts in response to climate change, rather than the
fine or annual detail found in tree rings and ice cores. But the effect
of climate change is indistinguishable from that of fires or insect infestation.