Speakers
Chronis Tzedakis explaining the causes of long term climate change

Chronis Tzedakis explaining the causes of long term climate change

Chronis Tzedakis (UCL London)
Phil Jones (University of East Anglia, Norwich)

 

What can we learn from the causes of climate change?

One of the biggest challenges facing Quaternary scientists is understanding the causes of climate change through the Quaternary.  By understanding the factors which have driven climate to change in the past we can better understand global climate itself. This means that scientists can investigate feedbacks within the climate system, and how climate is likely to respond in the future.  This research is undertaken at a huge variety of different temporal scales, from investigating the drivers of 100,000 year glacial-interglacial cycles, to understanding the recent global warming “pause”.

What have been the major developments in our understanding of the causes of climate change?

The development in our understanding of climate change has been incremental, with researchers building on previous work.  One of the major turning points was the realisation of the true impact of factors such as orbital cycles and orbital variation on climate dynamics during the Quaternary.  Using these natural cycles of Earth’s climate we can now predict the onset and duration of an interglacial – with good accuracy – based upon whether obliquity and precession are in phase or not.

Detailed study of recent climate change allows and understanding of the forcings and feedbacks within the climate system, as over long timescales, forcings can turn into feedbacks.  By studying the impact of solar variability on global climate, it has been shown that between 1750 and 2011, solar increase is responsible for only 0.2 W/m3 of warming.  In contrast, anthropogenic warming is responsible for 2.3 W/m3.  These figures still have large error margins, but are clearly different by an order of magnitude.  Over the past 200 years, variation in the quantity of volcanic input has been a large factor in internal climate variability.  Increasing volcanism since 2000 could be responsible for the recent climate “pause”.  Large volcanic events are the largest single contributors to climate change.  However, they are very infrequent, meaning that their overall impact is low.

There has been much debate about the mechanisms of natural phenomena such as the North Atlantic Oscillation and the Atlantic Meridional Overturning Circulation, and how these will respond to climate change in the future.  These modes of variability are natural, so will continue to operate in the future.  Though we can investigate and predict how they will be modulated by anthropogenic climate change, there will not be “new modes” of variability.

 

 What are the major challenges still faced?

Despite the progress that scientists have made in understanding the causes of climate change, there remain some unanswered questions:

Chronis Tzedakis and Phil Jones discussing some of the causes of climate change

Phil Jones and Chronis Tzedakis during a Q&A session on climate change research

                    • There has been a huge increase in our understanding of orbital forcing, and its impact upon global climate.  However, there is still no single, unified theory of Quaternary climate change sufficient to explain all variability.  This remains a challenge for the future

 

                    • We understand other long-term changes, such are when and why the Mid-Pleistocene Transition occurred.  This was a switch from 41,000 year to 100,000 year interglacial/glacial cycles 1 million years ago.  However, we do not understand why the climate during 41,000 year cycles were symmetrical and during 100,000 year cycles the climate was saw-toothed.

 

  • Over shorter time-scales, there remain significant errors in identifying specific contributions to observed warming trends (solar, volcanic, land-use).  This makes it difficult to isolate the precise source of climate warming.  Over small spatial scales, for example, it is impossible to see if climate is following a natural or anthropogenic trend.  Detection of the anthropogenic warming signal is only currently possible at a global scale.  These problems are only likely to be solved through development of methodology.