A new reconstruction of global mean surface temperature trends over the past 2,000 years has identified the major causes of decade-scale climate change. The analysis suggests that the Earth’s current rate of warming, caused by human emissions of greenhouse gases, is greater than any previously observed rate of warming. The researchers also found that airborne particles from volcanic eruptions were primarily responsible for several brief episodes of global cooling before the Industrial Revolution in the mid-19th century.
The new temperature reconstruction is also broadly consistent with climate model simulations of the same period. The researchers found agreement for temperature changes caused by identifiable factors, such as volcanic aerosols and greenhouse gases, as well as random fluctuations in climate that occur on the same time scales. This suggests that current climate models accurately represent the contributions of various influences on global climate change – and are able to correctly predict future global warming.
The research team – 19 members of the Past Global Changes (PAGES) project, including University of Maryland associate professor of geology Michael Evans – used seven different statistical methods to perform the reconstruction. The results are published online on July 24, 2019, in the journal nature geoscience.
“Our reconstructions resemble the ‘hockey stick’ diagram of global temperature change that was first reconstructed more than two decades ago,” said Evans, who is also co-chair of PAGES and has a cross-appointment to the UMD Interdisciplinary Earth System Science Center (ESSIC). “Thanks to the work of the PAGES community, we now have a lot more data. The results were consistent no matter how we created the reconstructions or what randomly selected input data subset we did. used.”
The new 2,000-year-old reconstruction improves on previous efforts by using the most detailed and comprehensive database of its kind ever assembled. This dataset, painstakingly compiled by PAGES researchers, includes nearly 700 separate publicly available records from sources containing indicators of past temperatures, such as long-lived trees, reef-building corals, cores ice and marine and lake sediments. The data comes from all continental regions of the Earth and major ocean basins.
By comparing the new reconstructions with existing climate simulations generated using climate models from the Coupled Model Intercomparison Project 5 (CMIP5), the PAGES research team was able to determine the relative contributions of several influences on global temperatures over time. 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 man-made influence of greenhouse gas emissions.
The results suggest that volcanic activity was responsible for variations before about 1850. After that, greenhouse gases became the dominant influence on the global climate. By removing these influences from their analysis, the researchers also identified the magnitude of random changes that cannot be attributed to a specific cause. The team’s data-based reconstructions also agreed with the model simulations when assessing these random changes.
“This makes us more confident that our reconstructions are realistic and, in turn, that the climate models accurately simulate past and future global warming,” Evans added.
This agreement between reconstructions based on the researchers’ data and CMIP5 simulations suggests that existing climate models can accurately predict future global temperature changes over the next few decades, according to Evans. However, these simulations are highly dependent on the choices humans will make in the future, which is very difficult to predict, Evans added.
“The uncertainty about the influence of human activities is not that great if you look at just a few decades,” Evans said. “But in the longer term, the choices we make about our energy sources and the amount of carbon those sources emit really matters.”
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