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The warming trend observed over the past century and models of its future trajectory suggest that we are

rapidly heading towards a climate that was last experienced more than three million years ago during the Pliocene. Climate reconstructions for this time, based on proxy measurements from geological archives, reveal that atmospheric carbon dioxide concentrations were ~400 ppm (refs 1,2) — comparable to present-day levels — and average surface temperature was 2–3 °C warmer than at present3,4. Furthermore, the configuration of the oceans, continents and ice sheets was broadly similar to today. Given the magnitude of the model-based climate projections for the end of this century, and the difficulty in assessing their accuracy, scientists are increasingly looking back to the future for appropriate climate analogues. However, a challenge to reconstructions of Cenozoic surface temperatures has been the limited number and uneven geographical distribution of palaeo-observations. Writing in Nature Climate Change, Dowsett et al.4 provide the most comprehensive compilation so far of Earth’s surface temperatures for the warm climate period of the Pliocene between 3 and 3.3 million years ago.

The researchers present their data set as a basis for testing or ‘ground truthing’ the performance of an ensemble of global climate model simulations. The effectiveness of this approach, commonly referred to as ‘hind-casting’, is dependent on the confidence that can be placed on the palaeo-temperature estimates. Palaeo-temperature reconstructions are based on proxy measurements of a change in chemical, physical or biological parameters that reflect, in a quantitative and well-understood manner, the temperature change in the environment where the proxy carrier existed.

There is little consistency in the way uncertainties are reported for proxy climate estimates in the literature. In most cases, reported error bars represent the analytical and/or calibration error, with the calibration often assumed to be correct and the assumptions involved in the proxy technique

usually unquantified. Dowsett et al.4 take a holistic approach to assessing confidence in their Pliocene surface temperature estimates, which are based on three well-established and commonly used proxies: (1) the UK

37 index of fossilized organic molecules from marine algae; (2) the Mg/Ca ratio in the fossilized calcite shells of mixed-layer planktonic foraminifera; and (3) extant planktonic microfossil assemblages (most commonly foraminifera, but also diatoms and radiolarians) and their relationship to modern analogues.

The researchers use a quality score on an arbitrary scale to assign confidence to both the quantifiable methodological uncertainties, as well as the non-quantifiable, mainly environmental, factors. For the assessment of the latter they use ‘expert subjectivity’ that takes into account the quality of the age control, the number of samples, fossil preservation and abundance, performance of the proxy method used and agreement of multiple proxy estimates. The summation of the individual scores for each attribute provides an overall confidence score for the temperature reconstruction of a particular site. This confidence estimate is then expressed as a verbal qualifier (for example, very high,

high, medium, low) using a similar approach to the ‘calibrated’ uncertainty language used by the Intergovernmental Panel on Climate Change. Their approach allows more rigorous evaluation of individual proxy temperature reconstructions, leading to an improved ability to assess past climate models.

A comparison of the Pliocene marine surface temperatures, newly characterized in terms of their confidence level, with the results of the global climate model simulations shows good agreement between sea surface temperature reconstructions. Both methods also indicate that global mean annual surface temperature was ~2 °C above the pre-industrial mean3,4. Figure 1a illustrates the Pliocene sea surface temperature anomaly derived from the researchers’ multi-model and proxy data for mean interglacial climate. The anomalies are calculated with respect to pre-industrial surface temperature.

A feature of global climate model and surface temperature proxy reconstructions is that, for warmer than pre-industrial (for example, Eocene and Pliocene) climate states, north–south temperature gradients are significantly reduced4,5. This extratropical warming, known as polar amplification, is

PALAEOCLIMATE

Looking back to the futureFirmly establishing Earth’s surface temperatures during a sustained episode of global warming in the Pliocene will help ‘ground truth’ projections of future climate based on computer simulations using global climate models.

Tim Naish and Dan Zwartz

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Figure 1 | Comparison between palaeoclimate proxy and climate model reconstructions of sea surface temperature for the warm Pliocene, 3.3 to 3 million years ago, as reported in Dowsett et al.4. a, Proxy (circles) and multi-model mean sea surface temperature anomalies calculated relative to the pre-industrial value. b, Extratropical amplification is calculated from the multi-model mean (black line) and proxy (circles) sea surface temperature anomalies normalized to the global mean temperature anomaly. Shaded band indicates two standard deviation uncertainty.

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318 NATURE CLIMATE CHANGE | VOL 2 | MAY 2012 | www.nature.com/natureclimatechange

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about two times the global mean (Fig. 1b). Climate models seem to underestimate the strength of this amplification with respect to proxy-based reconstructions by 30–50%. This is particularly so for the Northern Hemisphere. However, for the Pliocene there is reasonable agreement between model- and proxy-derived Southern Hemisphere gradients. Polar amplification is also suppressed in meridional sea surface temperature gradients compared with surface air temperature gradients owing to the presence of high-latitude sea ice in the pre-industrial control, which places a lower limit on sea surface temperature.

The substantial extratropical amplification has implications for the stability of polar ice sheets, suggesting that they may be highly sensitive to relatively small increases in carbon dioxide forcing6, surface air temperature7 and oceanic temperature8,9. For the Pliocene, a picture is developing of a world where there was much less polar ice and global sea level was 10–30 m higher than at present. This is based on geological reconstructions10,

glacio-hydro-isostatic modelling11 and simulations of the Antarctic8 and Greenland7 ice sheets.

Climate reconstructions for the warm Pliocene also provide an opportunity to explore the long-term climate sensitivity at current and near-future concentrations of atmospheric carbon dioxide. ‘Earth-system sensitivity’ is defined as the mean surface temperature increase resulting from a doubling of pre-industrial atmospheric carbon dioxide after slow feedbacks, such as those associated with polar ice sheets and ocean circulation, have played out. Sensitivities of 4–8 °C from a limited number of studies so far2,12, imply that Earth-system sensitivity may have a higher value than more traditional calculations of climate sensitivity from models, which are typically between 2 and 4.5 °C (ref. 13).

As the next Intergovernmental Panel on Climate Change assessment nears, scientists are looking to geological climate data to assess computer simulations of Earth-system responses to levels of carbon dioxide projected for the coming centuries. Given

the difficulty of verifying models in the Earth sciences, palaeo-observations with robust confidence assessments, such as the Pliocene global surface temperature data set reported in Dowsett et al.4, will make a significant contribution to this task. ❐

Tim Naish and Dan Zwartz are at the Antarctic Research Centre, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand. e-mail: Timothy.Naish@vuw.ac.nz

References1. Seki, O. et al. Earth Planet. Sci. Lett. 292, 201–211 (2010).2. Pagani, M., Liu, Z., LaRiviere, J. & Ravelo, A. Nature Geosci.

3, 27–30 (2010).3. Dowsett, H. J. et al. Stratigraphy 7, 123–139 (2010).4. Dowsett, H. J. et al. Nature Clim. Change 2, 365–371 (2012).5. Huber, M. & Caballero, R. Clim. Past 7, 603–633 (2011).6. DeConto, R. M. et al. Nature 455, 652–656 (2008).7. Dolan, A. M. et al. Palaeogeogr. Palaeoclimatol. Palaeoecol.

309, 98–110 (2011).8. Pollard, D. & DeConto, R. M. Nature 458, 329–332(2009).9. Mackintosh, A. et al. Nature Geosci. 4, 195–202 (2011).10. Miller, K. et al. Geology http://dx.doi.org/10.1130/

G32869.1 (2012).11. Raymo, M. E., Mitrovica, J. X., O’Leary, M. J., DeConto, R. M. &

Hearty, P. J. Nature Geosci. 4, 328–332 (2011).12. Lunt, D. J. et al. Nature Geosci. 3, 60–64 (2010).13. Knutti, R. & Hegerl, G. C. Nature Geosci. 1, 735–743 (2008).

Studies investigating changes in glacier mass-balance over the past century reveal a consistent picture of glacier

shrinkage in most regions of the world1. In glacierized river catchments, glacier shrinkage alters river discharge by reducing glacial meltwater inputs and proportionally boosting groundwater contributions to river flow. These changes in river dynamics alter habitat conditions, potentially threatening the unique organisms that live in these proglacial rivers. However, the scale of these effects on biodiversity remains poorly quantified. Writing in Nature Climate Change, Dean Jacobsen and co-workers2 address this research gap by analysing how three key biodiversity measures — local taxa richness, taxon turnover and regional taxa richness — have responded to the shrinking of mountain glaciers. Based on macroinvertebrate (mostly insect larvae) data from more than 100 sites — distributed across the Ecuadorian Andes, the Swiss and Italian Alps, and the

Coastal Range Mountains in southeast Alaska — they predict the consequences of the retreat and disappearance of glaciers on aquatic biodiversity2.

The study by Jacobsen et al. indicates that 11–38% of the regional species pool can be expected to be lost following complete disappearance of glaciers in the catchment. This is a plausible scenario given that models suggest that a further 3 °C increase in global air temperature would result in the loss of most alpine glaciers3. Nevertheless, even more-moderate changes that result in a steady shrinkage of glacier mass are found to be likely to reduce taxon turnover in proglacial river systems and local richness at downstream reaches where glacial cover in the catchment is less than 5–30%. The study emphasizes not only the vulnerability of local biodiversity hotspots, but also that the number of species lost could greatly exceed the few known endemic species found only in glacier-fed rivers.

Several studies in glacial rivers across the globe have shown that the flora and fauna follow a predictable pattern along the steep environmental gradients, with greater taxa richness and diversity with distance from the glacier and a predictable species turnover4. Two key environmental parameters that are shown to strongly effect life in glacial streams are water temperature — which is very cold (below 1 °C throughout the year) at the glacial snout and increases downstream — and channel stability, which is very low (highly unstable, moving substrate) close to the glacier, becoming more stable with increasing distance from the glacier. In their comparison of glacial river systems, Jacobson et al. confirmed that local richness was a function of glacial cover — being lowest at highly glacierized sites. However, they also found that local richness peaked at 5–30% glacial cover where it was, contrary to conventional theory, even higher than sites with no glacial influence.

FRESHWATER ECOLOGY

Melting biodiversityGlacial meltwater contributions to rivers are declining in many parts of the world, but the effect of these changes on river communities remains poorly understood. Now a quantitative analysis points to the potential scale of this biodiversity problem.

Leopold Füreder

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