New study explains the cause of multidecadal climate changes

A new reconstruction of global average surface temperature change over the past 2000 years has identified the main causes for decadal-scale climate changes. The result shows that the Earth's current warming rate, caused by human greenhouse gas emissions, is higher than any warming rate observed previously. The researchers also found that airborne particles from volcanic eruptions were primarily responsible for several brief episodes of global cooling before the Industrial Revolution of the mid-19th century.

This new temperature reconstruction also largely agrees with other climate model simulations for the same period of time. The researchers found agreement for changes in temperature caused by identifiable factors, such as volcanic aerosols and greenhouse gases, as well as random fluctuations in climate that took place on the same timescales. This suggests that current climate models accurately represent the contributions of various factors that influence global climate change and are capable of correctly predicting future climate warming.

The research team working on the Past Global Changes (PAGES) project used seven different statistical methods to perform the reconstruction and these results were published online July 24, 2019 in the journal Nature Geoscience. The new 2,000-year reconstruction improves on previous efforts by using the most detailed and comprehensive database compiled by PAGES researchers. The dataset includes nearly 700 separate publicly available records from sources that contain indicators of past temperatures, such as long-lived trees, reef-building corals, ice cores, and marine and lake sediments. The data are sourced from all of Earth's continental regions and major ocean basins. 

​Graph shows global mean rates of temperature change over the last 2,000 years, as determined by a new reconstruction based on climate proxy data. Red denotes temperature increases while blue denotes temperature decreases. The green line shows the maximum expected warming rate without human influence; the dashed orange line signifies the ability of climate models to simulate this natural upper limit. The black line indicates average global as determined by direct measurements since the Industrial Revolution. Credit: University of Bern
 
By comparing the new reconstructions with existing climate simulations generated using the Coupled Model Intercomparison Project 5 (CMIP5) climate models, the PAGES research team was able to determine the relative contributions of several influences on global temperatures over time. These included natural influences, such as fluctuations in solar heating and the cooling effect of particles ejected by volcanic eruptions, as well as the human-caused influence of greenhouse gas emissions.
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The results suggest that volcanic activity was responsible for variations before about 1850.Thereafter, greenhouse gases became the dominant influence on global climate. By removing these influences in their analysis, the researchers also identified the magnitude of the random changes that cannot be traced to a specific cause. The team's data-based reconstructions also agreed with model simulations when evaluating these random changes. This agreement between the researchers' data-based reconstructions and the CMIP5 simulations suggests that existing climate models can accurately predict future global temperature change over the next few decades. However, these simulations depend heavily on the choices that humans make in the future, which is very difficult to predict. According to the researchers, the uncertainty in the influence of human activities is not so large when looking forward only a few decades but in the longer term, the choices that are made regarding energy sources and how much carbon these sources emit will greatly matter.
 
Journal reference: Consistent multidecadal variability in global temperature reconstructions and simulations over the Common Era, Nature Geoscience (2019).  DOI: 10.1038/s41561-019-0402-y

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© 2019 Oceanographer Daneeja Mawren

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