Robert A. Spicer

Uncertainty in predictions of the climate response to rising levels of greenhouse gases.

The range of possibilities for future climate evolution needs to be taken into account when planning climate change mitigation and adaptation strategies. This requires ensembles of multi-decadal simulations to assess both chaotic climate variability and model response uncertainty. Statistical estimates of model response uncertainty, based on observations of recent climate change, admit climate sensitivities—defined as the equilibrium response of global mean temperature to doubling levels of atmospheric carbon dioxide—substantially greater than 5 K. But such strong responses are not used in ranges for future climate change because they have not been seen in general circulation models. Here we present results from the ‘climateprediction.net’ experiment, the first multi-thousand-member grand ensemble of simulations using a general circulation model and thereby explicitly resolving regional details. We find model versions as realistic as other state-of-the-art climate models but with climate sensitivities ranging from less than 2 K to more than 11 K. Models with such extreme sensitivities are critical for the study of the full range of possible responses of the climate system to rising greenhouse gas levels, and for assessing the risks associated with specific targets for stabilizing these levels.

Constant elevation of southern Tibet over the past 15 million years.

The uplift of the Tibetan plateau, an area that is 2,000 km wide, to an altitude of about 5,000 m has been shown to modify global climate and to influence monsoon intensity. Mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the Tibetan plateau, but the precise history of the uplift of the plateau has yet to be confirmed by observations. Here we present well-preserved fossil leaf assemblages from the Namling basin, southern Tibet, dated to ∼15 Myr ago, which allow us to reconstruct the temperatures within the basin at that time. Using a numerical general circulation model to estimate moist static energy at the location of the fossil leaves, we reconstruct the elevation of the Namling basin 15 Myr ago to be 4,689 ± 895 m or 4,638 ± 847 m, depending on the reference data used. This is comparable to the present-day altitude of 4,600 m. We conclude that the elevation of the southern Tibetan plateau probably has remained unchanged for the past 15 Myr.

The formation and interpretation of plant fossil assemblages

Publisher Summary Every fossil represents evidence that an organism lived, not in isolation, but in the context of a physical setting populated by other organisms. To understand the history of life, it is essential that the organisms represented by fossils are studied in their proper context—as part of a network of interacting physical and biological agencies that shape the lives of individuals and influence the contribution of those individuals to the processes and patterns of evolution. The environment in which an individual exists is determined not just by contemporaneous conditions, but also by physical and biological circumstances that have preceded it. Palaeontologists must therefore adopt a distinctly four-dimensional view of the world, spanning geological and biological disciplines. In recent years a major change in emphasis in palaeontology has been away from the study of individual isolated specimens. Instead a more integrated biological and geological framework is being developed.

Late Cretaceous–early Tertiary palaeoclimates of northern high latitudes: a quantitative view

Analyses of plant community structure, vegetational and leaf physiognomy, and growth rings and vascular systems in wood provide qualitative and quantitative data that can be combined to define non-marine palaeoclimatic parameters with better resolution than is available from other, principally sedimentological, methods. Application of these techniques to Cenomanian through Paleocene floras from high palaeolatitudes (75°-85°N) indicates a polar light regime similar to that of the present. Plant data suggests Cenomanian sea level mean annual air temperatures (MATs) of 10 °C, and MATs of 13 °C, 5°C and 6-7 °C in the Coniacian, Maastrichtian, and Paleocene respectively. Evapotranspirational stresses at sea level were low and precipitation was in most part uniform throughout the growing season in the Cenomanian, with possible seasonal drying occurring by the Maastrichtian. Maastrichtian winter freezing was likely, but periglacial conditions did not exist at sea level. Permanent ice was likely above 1700 m at 75°N in the Cenomanian, and above 1000 m at 85°N in the Maastrichtian. These near-polar data provide critical constraints on global models of Late Cretaceous to early Tertiary climates.

Late Cretaceous terrestrial vegetation: A near-polar temperature curve

Quantitative estimates of terrestrial paleotemperatures near the North Pole were derived from the physiognomy of the vegetation, including leaf-margin analysis, from rocks of latest Albian and Late Cretaceous age of the North Slope of Alaska. During the latest Albian and Cenomanian, mean annual temperature was approximately 10 ±3 °C. During the Coniacian, mean annual temperature may have been 2-3 °C warmer than during the Albian-Cenomanian but no higher than 13 °C. During the Campanian and Maastrichtian, mean annual temperature would have been about 2-8 °C. Although the ranges for the individual estimates overlap, differences among the floras indicate that the relative changes in mean annual temperature did occur.

A review of terrestrial and marine climates in the Cretaceous with implications for modelling the "Greenhouse Earth"

From the unique perspective of the geological record, it appears that the ‘Greenhouse Earth’ was a feature of climate for up to 80 % of the last 500 Ma, and that therefore our present glacially dominated climate is an anomaly. The Cretaceous in particular was a time of global warmth, an extreme greenhouse world apparently warmer than our current Earth. The geological record provides perspective and constraints against which the success of climate models can be evaluated. At present there are no ways of evaluating model predictions for the future of our ‘Greenhouse Earth’ until after the event. Retrodicting the past is therefore a very useful way of testing model sensitivity and robustness. The geological record tells us that the characteristics of the Cretaceous greenhouse world were a shallower equator-to-pole temperature gradient, shallow, well-stratified epicontinental seas with a tendency towards periodic dysaerobism, and a well-developed terrestrial flora extending to the high latitudes. Both marine and non-marine data show a global cooling trend throughout Late Cretaceous time, a trend that seems to correlate with declining atmospheric carbon dioxide.

Paleobotanical evidence for cool north polar climates in middle Cretaceous (Albian-Cenomanian) time

Mid-Cretaceous (Albian-Cenomanian) floras are abundant and diverse on the North Slope of Alaska. The older floras consist of conifers, cycadophytes, ferns, ginkgophytes, and sphenophytes (horsetails). Angiosperms appeared in latest Albian time and rapidly diversified. The preserved floras consist entirely of deciduous plants, with the exception of a microphyllous conifer, ferns, and sphenophytes. Deciduousness is evidence for strong seasonality, which for these floras might be variations in either light or temperature or both. Cool temperatures are suggested by the prevalence of toothed leaves among the angiosperms and the presence of large-leaved conifers. The paleobotanical evidence points to a mid-Cretaceous climate that was no warmer than cool temperate on the North Slope of Alaska.

Climate change and the evolution of high-latitude terrestrial vegetation and floras

Our understanding of polar vegetation and climate through time has expanded enormously in the past five years as a consequence of improved logistics, detailed studies of plant fossils in their proper sedimentological context, and the development of sophisticated physiognomic methods for extracting the climate signal present in plant fossil assemblages. These revelations are particularly timely in that climate change is most strongly expressed at the poles, and polar conditions play a critical role in determining global climate. By studying the evolution and change in polar vegetation, valuable insights on possible future biotic responses to global warming can be obtained.

Quantitative palaeoclimate estimates from Late Cretaceous and Paleocene leaf floras in the northwest of the South Island, New Zealand

Three new plant macrofossil assemblages were collected from Late Cretaceous and Paleocene fluvio-lacustrine sediments of the Pakawau and Kapuni groups in the northwest of the South Island, New Zealand. Palaeoenvironmental interpretations were made from each locality and palaeoclimate was deduced from the dicotyledonous angiosperm leaf component of each flora. A latest Cretaceous (Pakawau Bush Road locality) flora yielded 58 different dicotyledonous leaf forms; the two Paleocene collections, Ians Tip and Pillar Point Track, included 23 and 28 dicotyledonous leaf forms respectively. Quantitative palaeoclimate estimates were obtained using both Leaf Margin Analysis (LMA) and the Climate Leaf Analysis Multivariate Program (CLAMP). Temperature estimates suggest that there was a slight cooling from the latest Cretaceous into the early Paleocene in the northwest Nelson region of New Zealand, supporting similar Southern Hemisphere palaeoclimate findings from Antarctic data. Consistency in temperature estimates using different methods, including LMA, multivariate leaf morphological analysis (CLAMP), oxygen isotope data, regional versus local studies and global palaeoclimate models, suggests that the mean annual temperature for the Pakawau region in the latest Cretaceous was between 12 and 15°C. LMA produced temperature estimates between 6.5 and 8°C for the two Paleocene assemblages whereas CLAMP-produced estimates were slightly higher between 9 and 12.5°C

Cretaceous phytogeography and climate signals

We address two aspects of Cretaceous plants and climate. Firstly, we briefly characterize Cretaceous global phytogeography and review some quantitative temperature estimates derived from plants. Secondly, by adopting a multivariate statistical approach to palaeophytogeographic mapping, we examine the effect of the rise in diversity and ecological radiation of angiosperms on the climate signal offered by non-angiosperms.