We already know that this decade has been the warmest since modern records began in around the 1880s. But what about over the longer term? Researchers have been able to reconstruct the past temperatures within Europe for every year stretching back to the Roman Empire. And what they found was that the last three decades have probably been the warmest over that 2,000-year period.
By using tree-ring analysis, as well as historical records from doctors, priests and monks, the international team of 45 researchers from 13 countries determined that the summers during the last 30 years were on average 1.3°C (2°F) hotter than they were two millennia ago, and that heat waves have become longer, more frequent, and more severe. Not only that, but they suggest that current climate models underestimate past natural variability, meaning that the effects of climate change may be greater than predicted.
“We now have a detailed picture of how summer temperatures have changed over Europe for more than two thousand years and we can use that to test the climate models that are used to predict the impacts of future global warming,” explains Professor Danny McCarroll, one of the many coauthors of the study, published in Environmental Research Letters.
Using tree rings to reconstruct past temperatures is tricky, and much of the tree-ring data for before 755 C.E. comes from pine tree species that live in Finland, Austria and Sweden, which only grow during the warmer months between June and August. This is why the researchers can only say that modern Europe has been having increasingly warm summers, but cannot give certainty about the entire year. After 755, the increase in pine trees in other parts of Europe, such as Spain, gives a better understanding of regional differences.
This data was able to show that warm summers were occurring up to around the third century C.E., with the climate cooling slightly over the next four hundred years, but followed by a relatively warm medieval period. Europe then went into what’s often referred to as the “Little Ice Age” from the 14th to 19th century, before the temperature got ramped up during the 20th. This, the scientists say, shows clearly how human-caused, or anthropogenic, climate change is driving up temperatures.
The researchers finally then tested current climate models for the past 2,000 years to see if they matched up with the tree-ring data. They found that the past natural variability was greater than the models predicted, which has implications for any forecasts for future temperatures. When the variability is altered to the correct levels, and the impact of climate change is added on top, the frequency and severity of heat waves is much higher than previously thought.