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Arctic temperature analysis over a thousand years

Greenland site. Sea, sea ice and snow covered mountains in the background.

A research team has created a picture of historical temperatures in the Arctic, spanning over a thousand years. A new technique and a recently published database of proxy data have been used in the research, and the results are an important contribution to understanding temperature changes now and in the future.

The Arctic is the area in the world that is heating up the fastest, through so-called Arctic amplification. New research shows that the Arctic amplification also existed naturally for a thousand years, before the anthropogenic emissions. Over the past millennium the temperature changes in the Arctic have been between 1.2 and 2.1 stronger than the changes in the average temperature over the entire northern hemisphere, according to this study.
- Arctic amplification can accelerate global climate change by emissions of methane and carbon dioxide from the thawing tundra. Understanding the phenomenon historically helps us to envision how temperatures will change in the future, Hans Chen, researcher at the Department of Physical Geography and Ecosystem Science at Lund University, and part of the research team, says to Lund University news (in Swedish).

New technique

With the help of proxy data from, among other things, ice cores, lake sediments and annual tree rings and a new technique called paleoclimatic data assimilation, we have succeeded in obtaining completely new information about how Arctic temperatures have changed over the past thousand years, says Hans Chen.

How does paleoclimatic data assimilation work?

- Paleoclimatic data assimilation combines proxy data with information from a climate model. The problem with only using proxy data to reconstruct for example temperatures, is that we only have data for tens to hundreds of places in the world, which creates large gaps in the reconstruction. With the help of paleoclimatic data assimilation, we can fill in the gaps based on the physics of the climate model. In this way, we can create a reconstruction that is consistent with both the physics of the climate model and the information from the proxy data.

What clear changes can be seen during the analyzed period, is there anything that stands out?

- We see that the strength of Arctic amplification has varied on time scales from decades to centuries. Further analyzes showed that the variations were correlated with a phenomenon in the climate system called the Atlantic Multidecal Oscillation (AMO), which describes natural variations in sea surface temperature in the North Atlantic. Furthermore, we found a downward trend in Arctic reinforcement during the last millennium until 1985, which was probably due to two factors: a general downward trend in AMO, and a stronger anthropogenic greenhouse effect at lower latitudes compared to higher latitudes during the 20th century. 

How reliable is this study, it must be difficult to reconstruct such a long period of time?

- Of course there are some uncertainties in both the proxy data and the upscaling of the information. We evaluated the temperature reconstruction partly by comparing the values with temperature measurements during the instrumental period, and partly by withholding randomly selected proxy data which was then used for verification. The evaluations show that the reconstruction is in good agreement with temperature data based on instrumental measurements, and that its quality is slightly lower but comparable with a climate reanalysis based on instrumental measurements of surface pressure (20th Century Reanalysis).

The study was conducted by researchers in a Swedish-Chinese study with researchers from LU, GU and the Chinese Academy of Sciences. The article "Arctic amplification modulated by Atlantic Multidecadal Oscillation and greenhouse forcing on multidecadal to century scales" is published in Nature Communications.



Arctic amplification

Arctic amplification means that temperature changes in the Arctic occur faster than the global or hemispheric average. The phenomenon has been well known since the late 1960s and is due to strong feedback processes in the Arctic.

The most common explanation for Arctic amplification is the albedo effect: warming in the Arctic causes sea ice to melt, resulting in the darker sea surface absorbing more solar energy in summer and releasing more heat to the atmosphere during late autumn and winter. However, there are other feedback mechanisms that research studies have highlighted.

Exactly which mechanisms are most important, and whether global warming caused by anthropogenic greenhouse gases has affected Arctic amplification, are still debated issues.

Hans Chen portrait photo

Hans Chen, researcher at the Department of Physical Geography and Ecosystem Science.