The 2023 summer shattered records in many parts of the world. This unparalleled heat is not just a fleeting anomaly, it’s a consistent trend indicative of the severe impacts of climate change driven by human activities.

A new study published by researchers Jan Esper, Max Torbenson, and Ulf Büntgen, meticulously puts this summer into context. By using a combination of observational data and climate proxies, they show that the summer heat was unprecedented for at least 2,000 years. In the Northern Hemisphere, recorded temperatures were already over 2 °C higher than the pre-Industrial average.

By now, hopefully, we all know that global temperatures are rising as a result of man-made greenhouse gases. This increase isn’t linear. You can still have some cold days or even cold years, but overall, the trend is clear: it’s getting hotter and hotter.

Naturally, as temperatures continue to climb, the odds of freakishly hot weather events also increase. This was obvious in 2023. The past year was the hottest in recorded history, and by a wide margin. It shattered records — and then shattered them again. The new study goes on to show that for thousands of years, our planet hasn’t been this hot.

“We are placing the 2023 temperature extreme into a long-term context, that is the last 2000 years,” said Esper in a briefing on the paper. “If you want to do that, you need annual resolved temperature reconstructions, and the only proxy that can provide that are tree rings.”

A growing problem

Tree rings are valuable tools for studying past climate. Each ring represents one year of growth, with the width and density of the rings varying based on environmental conditions such as temperature and precipitation. During favorable growing seasons, trees produce wider rings, while narrower rings form during stressful periods. By examining these patterns, scientists can reconstruct historical climate conditions, extending our understanding of climate variability over hundreds to thousands of years. This enables researchers to obtain a continuous, high-resolution record of past climate changes even before recordings existed.

In this study, Esper and colleagues combined hundreds of these measurements from nine regions. They found that the coldest summer was in the year 536, a during a period of volcanic activity that blanketed the Earth in a layer of dust and reflected much of the heat. The warmest pre-Industrial summer happened during the Late Roman Warm Period. This was a phase of relatively warm climate conditions in Europe and the North Atlantic region, occurring roughly between 250 and 400 CE.

But the 2023 summer blew past that. It exceeded it by over 0.5 °C — and it was 3.93 °C hotter than the coldest summer. The researchers also calculated the average summer temperature in the Northern Hemisphere in preindustrial times. They found that the summer of 2023 was 2.07 °C hotter than instrumental averages between 1850 and 1900 CE.

This is the first reconstruction that uses tree rings to go back 2,000 years, says Esper. The others are a bit shorter or use different types of proxies. The study is also remarkable because it highlights annual changes, which means the proxies are precisely dated.

Challenges in measuring heat

The temperature proxies only cover land masses, Torbenson said in the same briefing. It’s much more difficult to assess what the temperatures were like in the oceans in pre-instrumental times, and it’s also difficult to assess temperatures in the Southern Hemisphere, when there are fewer proxies.

There is a temperature reconstruction for the past 2,000 years done by the Intergovernmental Panel on Climate Change, the UN body that assesses scientific research on climate change — but that research uses much more varied proxies, some of which are not precisely dated. So, you may see a temperature indication, but you’re not sure exactly what year it’s from. In the tropics as well, where there is no winter, tree rings are much more difficult to analyze.

The authors acknowledge that the warming observed in this study is not directly applicable to global conditions. Nonetheless, they emphasize that their findings highlight the exceptional nature of current warming trends and underscore the critical need for immediate measures to reduce carbon emissions.

However, despite these minor uncertainties, one thing is clear: 2023 was the hottest year on record, with the hottest summer on record, for at least 2,000 years (and probably much more).

Tackling the climate problem

The unprecedented warmth has far-reaching implications for our understanding of climate change and the urgency of addressing it. These findings reinforce the need for immediate, sustained efforts to reduce carbon emissions and mitigate the impacts of global warming. The 2023 summer’s deviation from natural climate variability is a clear and severe reminder that our planet’s temperature is entering uncharted territory.

Tackling climate change is one of the biggest challenges in our society, if not the biggest. It requires a concerted effort from both society and individuals. On a societal level, governments and organizations must implement policies to reduce greenhouse gas emissions, transition to renewable energy sources, and invest in sustainable infrastructure. Meanwhile, on an individual level, we can all make an impact by adopting sustainable lifestyles: reducing energy consumption, using public transportation, recycling, and supporting environmentally responsible products and companies. Education and awareness are also crucial, as informed citizens are more likely to advocate for and participate in climate action.

The study was published in Nature.