Montana Climate Matters
Current climate change is different
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Have we ever seen anything like current climate change? It’s a fair question that geologists are often asked, but the answer isn’t straightforward.
In the past, very warm periods have been triggered by large-scale volcanic activity, releasing enormous amounts of greenhouse gases into the atmosphere. For example, volcanic eruptions during the mid-Cretaceous Thermal Maximum (120-80 million years ago) led to atmospheric carbon dioxide (CO2) levels that were 4-5 times higher and temperatures 22 degrees F warmer than today.
Millions of years of extreme heat brought dramatic ecological changes on land and in the ocean, which dinosaurs survived. Their extinction came 66 million years ago from a massive asteroid impact.
More recently, the Earth has experienced a series of cold and warm periods during the ice age of the last 2.6 million years. Air bubbles trapped in layers of Antarctic ice preserve the composition of the atmosphere at the time the layer formed. CO2 levels in the bubbles and other information from ice cores have been used to reconstruct the climate history of the last 800,000 years. These records show that cold, glacial periods consistently had low concentrations of atmospheric CO2, about 180 parts per million (ppm), and intervening warm, interglacial periods had levels of about 280 ppm.
Glacial-interglacial oscillations are caused by cyclical changes in Earth’s orbit and axial tilt that affect the amount of solar energy received by the planet. Small changes in incoming solar radiation dramatically alter Earth’s energy balance, which controls global temperature, weather patterns, and precipitation.
We live in the most recent interglacial period, which began about 18,000 years ago when rising atmospheric CO2 and solar radiation led to higher temperatures and glacial melting. Montana has experienced significant climate changes over this period.
Warming of 9-10 degrees F in the first 7000-8000 years of the interglacial transformed previously ice-covered landscapes into forests and grasslands. This warming also released glacially dammed lakes around Missoula and Great Falls; the ensuing, catastrophic floods carved the Channeled Scablands of eastern Washington and the Shonkin Sag east of Great Falls.
About 5,600 years ago, forest grew 600 feet above present treeline on the Beartooth Plateau, providing evidence that summers were warmer than present for several centuries. This information comes from analyzing the growth rings of ancient logs that were buried under an ice patch for thousands of years until recent warming.
In Yellowstone, prolonged drought from AD 1233-1362 caused Old Faithful to stop erupting, likely because there wasn’t enough snow to recharge subsurface waters. Future droughts and shrinking snowpack will probably have similar consequences for geysers.
Geologists often draw on the past to understand the present and the future, but there is little in the geological record to match current climate change. On glacial-interglacial time scales, we should be on a cooling trend heading into the next glacial period. Instead, we’re getting warmer with atmospheric CO2 concentrations of 425 ppm that exceed any level experienced in the last 3.5 million years. Unlike ancient periods with high CO2, the current rise in greenhouse gases has occurred with unprecedented speed in the last 150 years of the industrial era.
Climate certainly has changed throughout Earth’s history and for many reasons. The difference now is that the change is largely human caused and the impacts are unfolding over years and decades, not thousands or millions of years. In some ways, this is good news. Unlike the dinosaurs who had no control over their destiny, we understand the cause and impact of current climate change, and we know the solution. Appreciating the uniqueness of the present situation is an important step towards fixing the problem.
Cathy Whitlock is Regents Professor Emerita of Earth Sciences at Montana State University and lead author of the Montana Climate Assessment (https://montanaclimate.org).
