Two ice-core methods reach same time-lag conclusion

Findings published by European researchers in the journal Science show temperatures and atmospheric carbon dioxide amounts increased simultaneously during the last major warming period, confirming findings that ACE CRC paleo-climate expert Dr Joel Pedro published in 2012. The new results demolish one of the favourite arguments of climate change contrarians: that temperature rise causes CO2 increase and not the other way around. Mainstream scientists had maintained that while CO2 may not have initiated the end of the ice age it certainly played a central role in a feedback loop which drove further warming. What we can say now is that CO2 was likely involved in the warming out of the ice age right from the start. “The take home message from Parrenin et al is that temperature and CO2 increased in lock step during the warming out of the last glacial period,” Dr Pedro said. “We came to the same conclusion using independent methods and different ice cores last year in a paper published in Climate of the Past.” Dr Pedro said that two studies by different groups of authors using different methods and several different ice cores had now come to the same key result: that Antarctic temperature and atmospheric CO2 increased essentially in synchrony as the Earth warmed out of the most recent glacial period, 20,000-10,000 years ago. It is still a big challenge to pinpoint exactly what natural processes were responsible for such a tight feedback process at this time. Dr Pedro said scientists in the carbon cycle and climate modelling communities working on this problem should be aided by the improved observations from the ice cores. He provides more detail and background: “A technical barrier confronting studies like Parrenin et al and ours from last year is that there is an inbuilt time offset in all ice cores between the CO2 and temperature record. The offset arises because temperature information is encoded at the surface in the composition of the water molecules in the snow itself, whereas the CO2 is captured at around 60-120m deep as the pores in the snow close off. This size of the offset, which we call ‘delta-age’, depends on snowfall and temperature and can range from a few decades to over a thousand years. We must correct for delta-age in order to provide an accurate assessment of the real past time relationship between temperature and CO2. For many cores the reported uncertainties in delta age were previously so large as to swamp results on the temperature and CO2 relationship. In Pedro et al we worked around this problem by using high snowfall ice cores from close to the Antarctic coast for which the delta age is much smaller. Parrenin et al use cores from central Antarctica with larger delta age, but use a completely new method that appears to greatly reduce uncertainties for those cores, and correct an earlier bias which consistently overestimated it. "Finally, it is important to point out that coupled rise in temperature and CO2 that marked the end of the glacial period occurred gradually, over about 8000 years. In contrast what we have seen since the start of the industrial revolution is a similar CO2 increase occurring over only a few hundred years. This is way faster than anything in the ice core record and it’s clearly human-caused. Just as the steady increase in CO2 helped to melt the ice caps and warm the Earth out of the glacial, the rapid increase now in CO2 is also driving up temperatures, only at a much faster rate.” Pedro et al media release

Authorised by the CEO of the Antarctic Climate and Ecosystems Cooperative Research Centre October 2019.

The ACE CRC was established and supported under the Australian Government’s Cooperative Research Centres Program.

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