The GISP2 ice core, along with other Greenland cores, provides a record of temperature variations over millennia. This is inferred from the oxygen isotope ratios (δ18O) in the ice, which act as a proxy for past temperatures.
Watch as the analysis from this three kilometers deep ice core reveals that, as luck would have it, we have appear to have used the coldest years of the last ten thousand years as our base level for temperature. The date coincides with the creation of modern-type climate thermometers and the passion for recording daily temperatures. So, of course we are warming back up.
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Studies using ice core data show that Greenland has experienced significant temperature fluctuations over the past 10,000 years or so. There were periods where temperatures were higher than today, like during the Holocene Climatic Optimum around 9,000 to 5,000 years ago.
The claim that temperature measurements began at the “lowest point in the cycle” might stem from the fact that instrumental temperature records only go back about 150 years, which aligns with a relatively cooler period post the Little Ice Age, a cold period that ended around the mid-19th century. This period could be seen as “lower” compared to earlier warm periods in the Holocene, but not necessarily the lowest point in the entire 10,000-year record.
The transition from proxy data (like ice cores) to instrumental data (thermometers) does indeed coincide with a time when proxy records suggest Greenland was cooler than during some earlier millennia. However, this doesn’t mean we are at the absolute lowest temperature of the entire cycle; it’s more reflective of where we were in the ongoing natural climate variability when modern measurements started.
Studies like those by Vinther et al. (2009) and others provide reconstructions showing that the last few centuries were indeed cooler in Greenland compared to earlier times in the Holocene, but these reconstructions also highlight that current temperatures, when including both instrumental and proxy data, are now rising towards or possibly exceeding those of the Medieval Warm Period or the Holocene Climatic Optimum in some regions.
The Greenland ice core data does tell us about long-term temperature cycles, showing that Greenland has been both warmer and cooler in the past than it is today. The claim that we started measuring temperatures at the “lowest point in the cycle” perhaps oversimplifies the complex nature of climate variability over millennia, but it’s a great place to start.