Leverhulme Climate Symposium 2008

Lonnie G. Thompson

Glaciers are among the first responders to global warming, serving both as indicators and drivers of climate change.  Over the last 30 years we have been engaged in a program of systematic recovery of ice cores from tropical glaciers.  The resulting climate records, along with other proxy data (indirect indicators), have produced three primary lines of evidence for past and present abrupt climate change.  First, the current warming at high elevations in the mid- to lower latitudes is unprecedented for at least the last two millennia.  Second, the continuing retreat of most mid to low-latitude glaciers, many having persisted for thousands of years, signals a recent and abrupt change in the Earth’s climate system.  Finally, there is strong evidence within and around glaciers for a widespread event about 5,200 years ago that marked the transition to cooler conditions that occurred through much of the world and coincided with structural changes in several civilizations.  Together, these three lines of evidence argue that the present warming and associated glacier retreat are unprecedented in many areas for at least 5000 years. 

The talk gave specific evidence of recent acceleration in the rates of ice loss on tropical glaciers.  The current melting of these ice fields confirm model predictions that temperatures in the tropics are changing more rapidly at higher altitudes.  The ongoing rapid, global-scale retreat of mountain glaciers is not only contributing to global sea level rise, but threatening fresh water supplies in many of the world’s most populous regions.  Along with the danger posed by ongoing climate change, the talk touched on the human response.

 

Peter M. Cox

       

We now know a good deal about recent climate change and its causes. Global average temperature has increased by about 0.7oC since the late 19th century, with most of the warming since 1980; and atmospheric CO2 concentration has increased by more than 25% (~100 ppmv) since 1700. Significant further climate change is projected in the coming decades, but its severity is uncertain, in part because of complex interactions between the physical, chemical and biological elements of the Earth system. The natural carbon cycle is currently performing an important service for humankind by absorbing about a half of anthropogenic CO2 emissions, leaving the other half of the CO2 emissions to accumulate in the atmosphere and add to the greenhouse effect. This “airborne fraction” has been relatively constant over the last few decades (aside from seasonal variations), but it may start to vary in the future because of climate effects on the land carbon cycle.

 

Models now suggest that climate-carbon cycle feedbacks will accelerate 21st-century climate change, but the magnitude of this acceleration varies markedly between models.   There is now an urgent need to reduce the uncertainties in climate-carbon cycle feedbacks through better use of observational data. This talk described how the combination of models and data, over a range of timescales, can help us to pin down climate-carbon cycle feedbacks. Particular attention was paid to evidence of two-way links between climate and CO2 in the rich records of the Earth’s past.

Lessons from the past concerning our climatic future

Jess Adkins

“Past climate change” is a phrase that means many things to many people.  This talk first introduced the spectrum of timescales that help us understand how palaeoclimate informs the understanding of our climatic future.  From tree rings and ice cores that overlap with the last hundred years or so of weather measurements, to ocean sediments stretching back tens of millions of years, it can be very hard to keep track of which piece of the geologic record is yielding new discoveries.  Several examples of past climate changes help us understand the scale of natural climate swings in the past.  The talk then put these in a modern context.  Finally, Prof. Adkins showed how palaeoclimate is unique in its ability to help us understand the mechanisms of climate change, both past and contemporary. 

The palaeoclimate record is crucial to our modern debate about how climate will evolve in the coming decades and centuries because it offers a window into the physics of how the system responds to change.  We look to the past not just for “analogs” that will accurately replay themselves, but more importantly for the fundamental physics of ocean-atmosphere-land-ice interaction that will govern the future.

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