Causes of change in European mean and extreme climate over the past 500 years

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Geosciences

Abstract

The aim of our project is to understand the causes of variability and change in mean and extreme temperature, and changes in mean precipitation over the last 500 years, largely focusing on Europe. Spatial patterns of climate change and changes in extremes are important since climate affects society and the natural world on regional scales. By understanding why temperature and temperature extremes as well as precipitation have varied in the past, we will be better able to predict how it might vary in the future. This will be because we will have improved our knowledge of what mechanisms were important in changes in the past, and tested the ability of climate models to reproduce what has happened in the past. Researchers have reconstructed European temperature variability over the last 500 years, and long homogenized instrumental records are now available that go back into the 18th century, and in some cases have daily resolution. Also, some reconstructions of precipitation and circulation are becoming available that go back reliably about 250 years, and with more limitations, 500 years. Other investigators have reconstructed changes in solar activity, the timing and magnitude of major volcanic eruptions and the composition of the atmosphere using data recorded in the ice sheets. The most important natural 'forcing' of climate has been thought to be changes in radiation from the Sun. This is reconstructed from measures of solar activity such as sunspots. However, another possibly more important climate forcing is the effect of large explosive volcanic eruptions. These inject sulphate aerosol into the upper atmosphere, which reflect more sunlight, causing cooling. Since the 18th and 19th centuries human forcings have become important. Human emissions of carbon dioxide and other greenhouse gases are well mixed, so cause roughly equal warming everywhere. From the 18th century on substantial deforestation took place. The removal of forest makes the land reflect more sunlight back to space, causing a cooling. The main fossil fuel burnt in the 19th century was coal, whose burning causes sulphur to be emitted to the atmosphere. The sulphur forms a short-lived aerosol which acts to cool regions close to where it is emitted. Thus, human forcings would be expected to have a rather complex regional affect with, in the Northern Hemisphere, warming from greenhouse gases being, partly to fully, offset by cooling due to deforestation and aerosols. To find causes of the observed patterns of climate change over Europe and the Northern hemisphere we will use climate models that can simulate what would have happened due to each individual 'forcing' described above. Then using mathematical techniques we will compare these 'fingerprints' with reconstructions of past climate change and climate variability and determine which changes have been caused by external influences, and which are just a representation of a naturally varying climate. We will mainly focus on patterns of temperature and precipitation change over Europe since 1500, but also explore the causes of changes over the entire millennium. The results of the comparison tell us the relative contribution of each forcing to past climate change, and how much change is left unexplained and may have occurred spontaneously due to chaotic variability in the climate. Since climate models, like the real world, have chaotic variability, we will use several simulations to isolate the predictable component. Using models and reconstructions, we will also explore the mechanisms responsible for the historical changes.

Publications

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Hegerl G (2011) Use of models in detection and attribution of climate change in Wiley Interdisciplinary Reviews: Climate Change

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Hegerl GC (2011) Climate change. Using the past to predict the future? in Science (New York, N.Y.)

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Iles C (2013) The effect of volcanic eruptions on global precipitation in Journal of Geophysical Research: Atmospheres

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Jones C (2011) The HadGEM2-ES implementation of CMIP5 centennial simulations in Geoscientific Model Development

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Luterbacher J (2016) European summer temperatures since Roman times in Environmental Research Letters

 
Description The effect of solar forcing on climate is currently a highly debated topic. This project has found that it plays only a minor role over the last millennium, a finding which implies that it is very unlikely that changes in the sun's energy has contributed a significant amount to the warming observed over the 20th century . At the same time we have calculated that a substantial fraction of climate change over the past few hundred years can be clearly associated with other external influences, with the majority of externally forced change in the pre-20th century era due to large volcanic eruptions and changes in the concentration of greenhouse gases.
The project has found that the resulting residual climate variability (i.e. the past changes in temperature not due to external forcings) shows trends that are smaller than those over the past 50 or 60 years. This is important as it shows that it is unlikely that internal forcing alone can be responsible for the recent observed warming trend, a research product highlighted in chapter 10 of the recent IPCC AR5 WGI report.
This project has also led to collaborations both national and international. This has resulted in several publications regarding such subjects as last millennium temperature variations, the effect of volcanoes on the climate, sea-level rise and whether tree-rings widths fully capture the effect on temperature by volcanic eruption.
We have produced new climate model simulation results covering the last 1000 years. These have been used to carry out the above research and have now been made publically available. One simulation has been made available as part of the Paleoclimate modelling Intercomparison Project.
Exploitation Route The next step is to investigate the influence of volcanic and solar forcing on smaller regions, a topic that is currently being carried out by us in collaboration with fellow researchers which should lead to a forthcoming publication.

Our findings regarding the impact of natural climate variations in the past can also be used for better prediction of the future impacts of changes in solar irradiance and volcanic forcing.

The simulations have all recently been made public. Since these are currently the most comprehensive set of singularly forced simulations covering the last 600 years they should prove useful to wide number of researchers who are interested in the effect of forcings on past climate variability.
Sectors Agriculture, Food and Drink,Environment

 
Description Publications resulting from this grant have been cited by the Intergovernmental Panel on Climate Change report (IPCC) 5th AR5; chapter 10; and have contributed to a figure (figure 10.19) produced by the PDRA on this grant.
First Year Of Impact 2013
Sector Environment,Government, Democracy and Justice
Impact Types Societal

 
Description Intergovernmental Panel on Climate Change (IPCC) Lead Author and member of Synthesis report writing team.
Geographic Reach Multiple continents/international 
Policy Influence Type Membership of a guideline committee
Impact PI was involved in Intergovernmental Panel on Climate Change (IPCC)'s fifth Assessment report, as a Lead Author and in the Synthesis report, released 2015, as a writing team member. These reports arguably form the basis of the Paris Agreement on climate change, and authors are chosen for their scientific contributions to the area they are chosen for. Hence this relates directly to my publications under these grants.
URL http://www.ipcc.ch
 
Description Climate Change: Science and Society 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Climate Change: Science and Society: This was a half day conference under the auspices of the Royal Society of Edinburgh. I was a coorganizer and speaker see website report. It was well attended and well received and communicated the state of climate science, impact and mitigation knowledge on climate change to the wider public
Year(s) Of Engagement Activity 2015
URL http://www.rse.org.uk/wp-content/uploads/2016/10/Climate-Change-Science-and-Society.pdf