Ice cores are cylinders of ice drilled out of an ice sheet or glacier. Most records come from Antarctica and Greenland, and the longest cores extend to 3km (1.86 miles) in depth. The oldest continuous records to date extend 123,000 years in Greenland and 800,000 years in Antarctica.

Ice cores contain information about past temperature, and about many other aspects of the environment. Crucially, the ice encloses small bubbles of air that contain a sample of the atmosphere – from these it is possible to measure directly the past concentration of gases (including carbon dioxide and methane) in the atmosphere.

By measuring the ratios of different water isotopes in polar ice cores, we can determine how temperature in Antarctica and Greenland has changed in the past. The oldest ice core we have was drilled by the European Project for Ice Coring in Antarctica (EPICA) from Dome C on the Antarctic plateau. It extends back 800,000 years and shows a succession of long, cold ‘glacial’ periods, interspersed roughly every 100,000 years by warm ‘interglacial’ periods (of which the last 11,000 years is the most recent).

We’ve already learned about the greenhouse effect and how an increase in CO2 concentration increases the global average temperature, and by using historic data from the ice cores we can directly observe this to be true.

This image is from the British Antarctic Survey at Dome C, and shows 800,000 years worth of data. It shows a clear correlation between rising CO2 levels and increase in the temperature.

From looking at the Ice Cores, we can see that going back 800,000 years, CO2 levels and global temperature levels have tracked each other very closely, but couldn’t the warming we’re already seeing be part of the natural cycle?

In 1958, the observatory Mauna Loa was built in Hawaii and has been taking atmospheric CO2 readings ever since. In June 2023, the Mauna Loa observatory took an atmospheric reading of 423ppm (parts per million.) This isn’t just the highest it’s been in human history, it’s the highest recorded in 800,000 years of data. According to scientists, the last time CO2 levels were this high was during the Pliocene Epoch, around 3 million years ago, when the Earth was several degrees warmer, sea levels were an estimated 50 feet (15 meters) higher than they are today, and forests grew as far north as the Arctic.

The diagrams to the right show the “Keeling Curve” (named after Charles David Keeling, a scientist at the Mauna Loa observatory.) This first image shows the CO2 levels since the observatory opened, and the second image shows the current CO2 levels as well as historic CO2 data from the dome C ice core.

If you look to the far right hand side of the graph, you can see that the current CO2 concentration is quite literally off the scale. This is not a natural event.

More information on the CO2 level and why this matters can be found in this article from Yale University.

The 10 hottest years on record have all occurred since 2005, and the past 5 years have been the hottest on record. There is overwhelming evidence that the average temperature is rising and wide agreement from many different scientific bodies.

The image below is from NASA, and shows agreement from 5 different organisations on the rising temperature, including NASA’s Goddard Institute and the UK Met Office Hadley Centre.

Temperatures are rising, and they are rising fast.

Please watch this short video from NASA which gives a bit more information on 2020, which was the joint hottest year on record.

There are some scientists who disagree with the notion of human caused climate change, however there is overwhelming consensus. A widely shared statistic is that 97% of climate scientists recognise that human caused climate change is real and happening right now.

There was even a co-authored study by the authors of seven climate acknowledgment studies that should settle this question once and for all. The two key conclusions from the paper are:

1. Depending on exactly how you measure the expert consensus, it’s somewhere between 90% and 100% that agree humans are responsible for climate change, with most studies finding 97% consensus among publishing climate scientists.
2. The greater the climate expertise among those surveyed, the higher the consensus on human-caused global warming.

There was also a study performed on the 3% of articles that denied the link of human actions and climate change, and this study found their research “filled with errors.” An excerpt from the paper reads:

“A common denominator seems to be missing contextual information or ignoring information that does not fit the conclusions, be it other relevant work or related geophysical data. In many cases, shortcomings are due to insufficient model evaluation, leading to results that are not universally valid but rather are an artefact of a particular experimental setup. Other typical weaknesses include false dichotomies, inappropriate statistical methods, or basing conclusions on misconceived or incomplete physics.”

We can measure atmospheric CO2 levels and average temperatures, but what other things would we expect to see if the world was in fact warming?

According to the US National Oceanic and Atmospheric Administration & US National Climatic Data Centre, we would expect to see the following happen:

• Glaciers will retreat
• Tree-lines will shift poleward and upwards
• Spring will start to come earlier
• Species will start to migrate poleward and upward
• Sea ice level will drop
• Sea level will rise
• Sea surface temperature will rise

We are seeing all of these phenomena. Please watch these videos, which will give you more information on some of these topics.