Global warming began earlier than we thought

Our climate is warming. Evidence for this is unequivocal and is portrayed in countless scientific graphs produced by organisations across the world.

Even the 2016 Olympic opening ceremony featured an eye-catching graphic of global temperatures spiralling to ever increasing levels during the 20th century.

We typically think of global warming in the context of a problem that developed in the second half of the 20th century, and which will pose a (very large) challenge for future generations. But this isn’t the full picture.

Our new study, published in Nature, has found that in some parts of the world the Industrial Revolution kick-started global warming as early as the 1830s.  

Asking the question “when did global warming begin?” seems like a pretty fundamental piece of the climate change story. But it turns out to be a question that can’t be answered by studying instrumental temperature records because these direct climate observations only became common towards the end of the 19th century.

Instead, our study used natural records to examine how temperatures changed in different parts of the world during the last 500 years.

These natural archives include tree rings and ice cores that record year-to-year changes in air temperature over the continents. Within the oceans, coral skeletons and layers of sediment can be used to trace temperature changes of the ocean surface. The 500-year perspective allows us to pinpoint exactly when the warming trends that we are witnessing today started.

What we have found is that in the tropical oceans and over northern hemisphere continents the earliest signs of warming developed around the 1830s to 1850s.

Warming took a bit longer to establish in the southern hemisphere. This is probably because of the large expanses of ocean here, which can pull heat out of the atmosphere and move it into deeper ocean layers.

The early onset of warming can be attributed to the initial rises in greenhouse gases caused by the Industrial Revolution. The turning point in our climate was probably also influenced by recovery from a massive volcanic eruption (Tambora) in 1815, which caused the infamous “year without a summer” in Europe. But our climate model testing demonstrates that you don’t need this added volcanic effect to explain the early development of global warming.

“We are already living in a world where dangerous climate change impacts await us.”

In the 1830s, the level of carbon dioxide in Earth’s atmosphere was 280 parts per million, a typical pre-industrial value. From this point, burning of fossil fuels caused carbon dioxide to gradually rise to 295 parts per million by the year 1900.

This 15 part per million increase in carbon dioxide levels during the 19th century is small by today’s standards. It now takes only seven years for our consumption of fossil fuels to add the same amount of extra carbon dioxide to the atmosphere. But even the early increases in atmospheric carbon dioxide had a small but measurable impact on our climate.

By stepping back and viewing the full picture of how our climate has changed since, and because of, the Industrial Revolution, we can see that we are now well and truly operating in a greenhouse world. Climate change is not just some problem for the future. We, and the ecosystems that support us, are living it right now.

The urgency for international action to curb climate change has produced the Paris Agreement which entered into force in November this year. The Paris Agreement lays out an ambitious target of trying to limit global warming to below 1.5 degrees Celsius in order to avoid dangerous changes in our climate.

Around the same time that world leaders were signing up to the Paris Agreement, scientists from NASA announced that 2016 is on track to be the new hottest year on record. Currently, 2016 is tracking at 1.3 degrees above our climate baseline—a baseline that comes from instrumental records beginning in the 1880s. But our work has shown that in many parts of the world warming was already underway by this time.

As we nudge ever closer to the guardrail laid out by the Paris Agreement, we need to heed the messages from the past, which tell us that we are already living in a world where dangerous climate change impacts await us.