What does the past tell us about abrupt climate change?

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JOHN FEGYVERESI

We now know that human activities such as carbon emissions from fossil fuels are currently causing our planet to undergo abrupt climate change. How worried should we be about the consequences of this rapid change?

My colleagues and I study past climate by looking closely at ice cores from the depths of Antarctic glaciers. Antarctica’s ice sheets date back over 800,000 years and provide a very detailed record of past climate. These data tell us exactly when there have been abrupt climate changes in the past, and we can go back to those times to learn some of the causes and consequences. What do these records tell us?

They tell us that past climate changes are not random. They all happen for specific reasons. They tell us that over the long term, the Earth’s climate responds to changes in the way the Earth orbits the sun. They show that our climate has changed in direct response to changes in the concentration of greenhouse gases in the Earth’s atmosphere. They show that past climate changed in response to major alterations in ocean currents, volcanic eruptions and large meteor impacts. These records show that some past climate changes occurred over long periods of time, while other climate changes were much more abrupt.

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An example of abrupt climate change occurred 12,800 years ago, when temperatures dropped by as much as 10°C (18°F) in just a few decades. It happened because a large freshwater lake the size of California suddenly spilled into the North Atlantic, breaking the global conveyor belt of ocean currents. This abrupt change led to an immediate decline of the Clovis culture and other hunter-gatherer colonies of that time. It has altered countless ecosystems and led to the death of many species unable to adapt. Looking through the history of the Earth’s climate, we find many examples like this.

Here in Arizona, there is a rich history of ancestral Pueblos living in the cliffs. At Walnut Canyon in Flagstaff, you can walk through the still-preserved dwellings and imagine what it would have been like for people living between 600 and 1400 AD (now known as CE). During this time (800–1250 CE), the Earth was going through the Medieval Warm Period. This period brought climate change and persistent drought conditions to the southwest (particularly in the nearby Mesa Verde region), which eventually drove out many cliff-dwelling populations.

Beyond ice core data, layers preserved in limestone rock formations provide a climate record stretching back even further in time. From these records we learned that 251 million years ago a massive increase in greenhouse gases, carbon dioxide (CO2) warmed the Earth by more than 10°C (18°F) . This led to one of the greatest mass extinctions ever known on Earth, known as the “Great Dying”. Meanwhile, a series of massive volcanic eruptions in the region now known as Siberia released a huge amount of CO2 into our atmosphere.

As we see with current climate change, more CO2 in the atmosphere causes more CO2 to be absorbed by the oceans, making the water more acidic. So, 251 million years ago, it led to widespread acidification and reduced oxygen in the world’s oceans. As a result, over 90% of all species on the planet have disappeared. Of course, new species evolved, but recovery of full ecosystem function took nearly 50 million years.

Today, governments and people around the world are all interested in where our current climate is headed and what will happen as our oceans continue to acidify. Recent direct measurements show that average global temperatures have already increased by 1.2°C (2.2°F) in recent decades and that greenhouse gas emissions are the main cause of this change. Given our current emission rates, we can expect to reach 5°C (9°F) of warming by the end of this century.

What we have learned from past studies is that even small or subtle climate changes can have enormous impacts on our planetary life support systems and on human civilizations. If we are to avoid significant and fatal impacts on society, it is imperative that we work collectively to reduce our greenhouse gas emissions as quickly as possible.

John Fegyveresi, PhD, Asst. Professor and Director of the Climate Science and Solutions Graduate Program, Northern Arizona University and Northern Arizona Climate Change Alliance, www.NAZCCA.org/volunteer.

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