ANNEX II: THE MIDGE THERMOMETER AND CLIMATE
CHANGE
Note of visit to Entomology Department,
Natural History Museum
1. The Committee visited the Natural History
Museum on 29 October 2002 and, as part of the tour, visited the
Entomology Department were the Museum's very comprehensive collection
of "bugs" was stored and studied (at least until the
completion of the Darwin Centre Phase II). Members were interested
in the application of the collection to modern science policy
and problems. The Natural History Museum's memorandum sets out
the core importance of its 70,000,000 specimens from the natural
world to taxonomy and systematic biology, but discussions with
the Keeper (head of department) Dick Vane-Wright, and Steve Brooks,
Research Entomologist, indicated additional value in the comprehensiveness
of the museum's collection.
2. The collection of non-biting midges (Chironomidae)
had been built up over the last 150 years to be one of the most
comprehensive such collections in the world. The original focus
of the collection was on the adult stages of the midges because
it was on the adults that the taxonomy of the group was based.
However, previous curators of the collection had also ensured
that the immature larval and pupal stages were also collected
and incorporated into the collection. In particular, the collection
included many microscope slides on which reared material had been
mounted. These slides included the cast skins of larvae and pupae
as well as the adult insects into which they developed. So the
immature stages were associated with positively identified adults.
It was only from the existence of this associated material that
the immature stages of chironomidae can be identified. The significance
of this historical diligence was explained as set out below.
3. When this material had been originally assembled
there was little interest in the larval stages of midges. However,
over the last decade it had become apparent from work done at
the museum that midge larvae were powerful indicators of climate
change. They were abundant and well-preserved in lake sediments
and most species had rather narrow climatic tolerances. By taking
thin slices of a lake sediment core changes in the species of
midge in that lake could be examined at 10-year resolution over
the last 15,000 years—back to the end of the last ice age—when
lakes in northern Europe first formed. The collection of midge
larvae, associated with the adult stage, in the museum's collection
allowed the identification of the midge species preserved in the
sediment slices.
4. By looking at the modern distribution of midges
in Europe the optimum temperature for each species could be determined.
Using this information the species assemblage of fossil midges
in any particular sediment slice yields the likely temperature
associated with that slice. The performance of this midge thermometer
was tested by using it to estimate the present-day temperature
of a lake from its modern midge population. The midge thermometer
had been found accurate to about 1 degree Celsius. Steve Brooks
expected that this margin was reducible to about 0.5 degrees Celsius
with a little more work.
5. Analysis of oxygen isotopes in ice cores from
Greenland had provided a high resolution record of relative temperature
changes over thousands of years. The trends shown in the ice core
records were closely reflected by the trends found in midge-inferred
temperatures from lakes in northern Europe. The added bonus of
the midge record was that it provided a quantitative estimate
of the temperature changes, and was considerably cheaper to obtain.
6. By using midges to quantify past climate change
the natural background variability of climate change since the
end of the last ice age can be assessed and compared to climate
change during the last 70 years. The instrumental record goes
back, at most, only 250 years and this was not long enough to
provide a complete picture of natural climate variability. Investigating
how climate has changed in the past helped to enable understanding
of climate feedback and forcing mechanisms and to better predict
how climate systems were likely to respond to global warming.
The midge-inferred temperatures could also be used to validate
the climate models that were used to predict future climate change
by testing their ability to estimate climate change in the past.
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