What do tree rings, pollen and corals have to do with climate change? The answer is they all help scientists dig deep into earth’s history to reconstruct past temperatures. A brand new analysis using these so-called climate proxies finds 1971 to 2000 was probably warmer than any other 30-year period in the last 1400 years.
Thermometers have only been measuring temperature since about the late 17th century. So to understand past climate, scientists use natural recorders of temperature change, including seafloor sediments, corals, fossilised pollen, ice cores and tree rings.
In a new study published today in the journal Nature Geoscience, a team of 78 researchers collected more than 500 proxy records across the world – and examined temperature changes continent by continent.
Long term cooling
The most consistent pattern found across all continents was a long term cooling trend over the past one to two thousand years, which ended in the late 19th century.
The researchers say a combination of an increase in volcanic activity, changes in earth’s orbit, land use changes and decrease in solar activity caused the cooling.
On top of the long term cooling trend, temperatures jumped around on timescales of a few decades to a few centuries. But the timing of the fluctuations wasn’t the same across all continents.
For example, the researchers couldn’t find a consistent worldwide signature for the Medieval Climate Anomaly, a period of warming between the 10th and 14th centuries, suggesting it was a regional rather than global event. Professor Heinz Wanner from the University of Bern and co-author on the paper tells Carbon Brief:
“We knew the spatial temperature structure is complex [but] we were surprised how complex this pattern [was] during the so-called Medieval Climate Anomaly.”
Similarly, the researchers found little evidence for a simultaneous worldwide temperature shift to distinctly mark the Little Ice Age – a period of cooling between about 1400 and 1850.
Ice cores, like this one from West Antarctica, are one example of a natural recorder of past temperature, known as a climate proxy. Source: Heidi Roop
Global warming since the end of the 19th century reversed the long term cooling trend, the researchers say. The data suggest the 20th century global average temperature was about 0.4 degrees Celsius warmer than the average for the preceding five centuries.
But there are important differences between regions. Warming in the northern hemisphere is twice as fast as the southern hemisphere. The Antarctic in particular shows weaker warming. The researchers suggest this is down to the continent being surrounded by ocean, which warms slower than land.
Overall, the data suggest average temperature across all seven continents between 1971 and 2000 was probably higher than any other 30-year period in nearly 1,400 years.
The new study is the first to look in detail at differences in temperature change between individual continents. As the researchers say in the paper:
“[This is important because] climate variability [at the continental scale] is arguably more relevant to ecosystems and societies than globally averaged conditions.”
Wanner tells us the new study allows a detailed look at the southern hemisphere, whereas previous studies have been “mainly eurocentric or focused on the Northern Hemisphere”. The proxies used in the new study also provide temperature information for each year – so the reconstructed record is more detailed than some previous studies over the time period it covers.
Looking in detail at how earth’s past temperature has changed helps scientists separate the effect of human activity from fluctuations that occur naturally – known as natural variability.
And knowing how natural and human-caused effects interact to influence global temperature is important for future projections of climate change, the researchers say.
There are still some uncertainties. For one thing, there aren’t enough proxy records for Africa – which means it’s absent from the temperature reconstruction. But as new and improved proxy records are found, this intriguing field of research is making rapid progress.
PAGES 2k network (2013) Continental-scale temperature variability during the past two millennia. Nature Geoscience. DOI: 10.1038/NGEO1797