GCMs are currently producing simulated climate pre-dictions for the Mesozoic and Cenozoic that compare favourably with the distributions of the geological climate proxies discussed above
Trang 1approach having been pioneered by John Kutzbach
and Eric Barron
General Circulation Models
GCMs use the laws of physics and an understanding
of past geography to simulate climatic responses
They are objective in character They require
power-ful computers to handle vast numbers of calculations
Nevertheless, it is now possible to compare the results
of different GCMs for a range of times and over a
wide range of parameterizations for the past, present,
and future (e.g in terms of predictions of surface air
temperature, surface moisture, precipitation, etc.)
GCMs are currently producing simulated climate
pre-dictions for the Mesozoic and Cenozoic that compare
favourably with the distributions of the geological
climate proxies discussed above They can be used
effectively to predict sites of oceanic upwelling and
the distribution of petroleum source rocks and
phos-phorites Models also produce evaluations of
param-eters that do not leave a geological record (e.g cloud
cover, snow cover) and quasi-parametric phenomena
such as storminess Parameterization is the main
weak-ness of GCMs (e.g palaeogeography,
palaeobathy-metry, sea-surface temperature, orography, cloud
behaviour), and model output for continental interiors
is still colder in winter than indicated by
palaeonto-logical data The sedimentary and palaeontopalaeonto-logical
record provides an important way of evaluating
GCMs, and this is important because the same GCMs
are currently being used to predict possible changes
in future climate
The outputs discussed below were generated by an
AGCM (HadAM3) and an OAGCM (HadCM3L),
which is currently state-of-the-art The model was
de-veloped at the Hadley Centre for Climate Prediction
and Research, which is part of the UK Meteorological
Office The GCM consists of a linked atmospheric
model, ocean model, and sea-ice model The horizontal
resolution of the atmospheric model is 2.5latitude and
3.75 longitude This provides a grid spacing at the
equator of 278 km north–south and 417 km east–
west The atmospheric model consists of 19 layers It
also includes a radiation scheme that can represent the
effects of minor trace gases Its land-surface scheme
includes a representation of the freezing and melting
of soil moisture The representation of evaporation
includes the dependence of stomatal resistance on
tem-perature, vapour pressure, and carbon dioxide
concen-tration There is an adiabatic diffusion scheme, to
simulate the horizontal mixing of tracers
The ocean model has the same spatial resolution as
the atmosphere model and 20 vertical layers, with a
time step of 30 min This contrasts with HadCM3
(the standard version of the Hadley centre OAGCM), which uses a horizontal resolution of 1.25 1.25
Palaeoclimate of the Mesozoic – Model Output and Geological Data
The Mesozoic Earth was an alien world, as illustrated here by reference to a Triassic GCM simulation and geological data Throughout the Mesozoic dense forests grew close to both poles, experiencing months
of continual daylight in warm summers and months of continual darkness in cold snowy winters Neither Triassic nor Jurassic oceanic sediments provide good evidence, but in the Late Cretaceous, from ODP (Ocean Drilling Program)/DSDP (Deep Sea Drilling Program) data, the ocean depths appear to have been warm (8C or more at the ocean floor), and reefs grew 10 further north and south than at the present time During the era the whole Earth was warmer than now by at least 6C, giving more atmos-pheric humidity and a greatly enhanced hydrological cycle However, from modelling studies, it seems that much of the rainfall could have been predominantly convective in character, often focused over the oceans The model output might help to explain geo-logical data suggesting that major desert expanses extended across the continents in low latitudes From the model, polar ice sheets are unlikely to have been present because of the high summer tempera-tures The model suggests the possibility of extensive sea ice in the nearly enclosed Arctic seaway through parts of the year, but there is as yet no proxy data against which such predictions may be tested The Tri-assic world was a predominantly warm world; the model outputs for evaporation and precipitation con-form well to the known distributions of evaporites, calcretes, and other climatically sensitive facies Triassic: Comparison of Model and Proxy Data – A Case Study
Modelled temperatures (Figure 2) A significant fea-ture of the Triassic Earth is that the landmasses were almost symmetrically distributed in a broad arc about the equator (see Mesozoic: Triassic) A major aspect of the modelled Earth is its overall warmth Despite tem-peratures plunging to 20C and below over Siberia in the northern-hemisphere winter and to similarly low values over southernmost Gondwana in the southern-hemisphere winter, the annual average temperature is subdued in these high-latitude areas because of the high summer values achieved there (ca 24C) These high summer values preclude the possibility of year-round ice and snow
PALAEOCLIMATES 135