Human-made climate change started to occur much earlier than previously thought, new research reveals. Current consensus holds that anthropogenic climate warming began to take hold significantly in the 1950s. However, taking the period as a start-point, say a team of researchers led by Jianping Duan of the Chinese Academy of Sciences in Beijing, reflects defects in earlier data quality rather than real-world conditions.
“It has long been speculated that the human influence on the climate may have started much earlier than the recent data-rich period,” the researchers write in a paper published in the journal Nature Sustainability.
“Because of the limitations of early instrumental observations and temporal variations in the strength of anthropogenic influence, combined with internal climate variability and changes in natural external forcing factors, it has always been difficult to detect and attribute human influences on earlier climate changes.”
One of the main hurdles to assessing climate change at historical distances beyond a few decades, the researchers explain, arise from the practice of using annual mean temperature change as the unit of measure.
Instead, Duan and colleagues turned to a more easily detectable measure known as temperature seasonality, which is simply the difference between average winter and summer temperatures for any given region. The difference, over time, is taken to be a measure of the annual temperature cycle (ATC). A weakening ATC – that is, a smaller difference between summer and winter conditions – is an indication of global warming.
The researchers used a number of verifiable sources of climate proxy records – including Tibetan Plateau tree-ring data and sulfate concentration from glacier ice-cores – and combined them with instrumental observations recorded in four regions within the northern hemisphere.
The numbers were the crunched using European climate records stretching back to the year 1500 CE, the Climatic Research Unit Temperature database compiled by the University of East Anglia in the UK, and the international Coupled Model Intercomparison Project Phase 5 (CMIP5).
The results revealed that anthropogenic warming started to have a significant effect on northern hemisphere climate regions in 1865, and on the Tibetan Plateau in 1872. The changes followed a “weak and insignificant strengthening” of the ATC between 1700 and 1865.
“There is no ATC proxy evidence available that is long enough to identify when the sustained and significant ATC weakening started in northeastern Asia, North America, the northern mid-latitudes, the northern high latitudes and the northern mid–high latitudes,” the researchers write.
“However, observations starting in 1851 show discernible weakening in the magnitude of the ATC in all of these regions. These results indicate that although the specific year when the magnitude of the ATC began weakening might not be identical among all regions, prominent ATC weakening has occurred widely since the late nineteenth century.”
Although a weakening ATC was recorded across the hemisphere, the human contribution was not consistent. The researchers found evidence to suggest that in the high northern latitudes, the effect was driven by a build-up of greenhouse gas (GHG) production. In lower latitudes the effect was generated by anthropogenic aerosol accumulation.
And although the findings reveal that global warming is a phenomenon that is a lot older, and thus a lot more deeply entrenched, than previously assumed, Duan and colleagues find cause for optimism.
“These results imply that a policy of reducing GHG emissions and air pollution can mitigate the anthropogenic weakening of the temperature seasonality,” they conclude.
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