The latest sobering climate report: A lead author explains what the warnings mean

Image: AdobeStock

[Editor’s note: The following article is republished from The Conversation.]

Humans are unequivocally warming the planet, and that’s triggering rapid changes in the atmosphere, oceans and polar regions, and increasing extreme weather around the world, the Intergovernmental Panel on Climate Change warns in a new report.

The IPCC released the first part of its much anticipated Sixth Assessment Report on Aug. 9, 2021. In it, 234 scientists from around the globe summarized the current climate research on how the Earth is changing as temperatures rise and what those changes will mean for the future.

We asked climate scientist Robert Kopp, a lead author of the chapter on Earth’s oceans, ice and sea level rise, about the profound changes underway.

What are the IPCC report’s most important overall messages in your view?

At the most basic level, the facts about climate change have been clear for a long time, with the evidence just continuing to grow.

As a result of human activities, the planet is changing at a rate unprecedented for at least thousands of years. These changes are affecting every area of the planet.

Line chart showing influence over time of different sources of warming. Only human-caused emissions are on the same trajectory as the actual temperature rise.
Humans produce large amounts of greenhouse gas emissions, primarily through fossil fuel burning, agriculture, deforestation and decomposing waste.
IPCC Sixth Assessment Report

While some of the changes will be irreversible for millennia, some can be slowed and others reversed through strong, rapid and sustained reductions in greenhouse gas emissions.

But time is running out to meet the ambitious goal laid out in the 2015 international Paris Agreement to limit warming to well below 2 degrees Celsius above preindustrial levels (2 C equals 3.6 degrees Fahrenheit). Doing so requires getting global carbon dioxide emissions on a downward course that reaches net zero around or before 2050.

What are scientists most concerned about right now when it comes to the oceans and polar regions?

Global sea level has been rising at an accelerating rate since about 1970, and over the last century, it has risen more than in any century in at least 3,000 years.

In the years since the IPCC’s Fifth Assessment Report in 2013 and the Special Report on the Ocean and Cryosphere in a Changing Climate in 2018, the evidence for accelerating ice sheet loss has become clearer.

Over the last decade, global average sea level has risen at a rate of about 4 millimeters per year (1.5 inches per decade). This increase is due to two main factors: the melting of ice in mountain glaciers and at the poles, and the expansion of water in the ocean as it takes up heat.

Ice sheets in particular are primarily responsible for the increase in the rate of sea level rise since the 1990s. There is clear evidence tying the melting of glaciers and the Greenland Ice Sheet, as well as ocean warming, to human influence. Sea level rise is leading to substantial impacts on coastal communities, including a near-doubling in the frequency of coastal flooding since the 1960s in many sites around the world.

Since the previous reports, scientists have made substantial advances in modeling the behavior of ice sheets. At the same time, we’ve been learning more about ice sheet physics, including recognizing the potential ways ice sheets can become destabilized. We don’t well understand the potential speed of these changes, but they have the potential to lead to much more rapid ice sheet loss if greenhouse gas emissions grow unchecked.

These advances confirm that sea level is going to continue to rise for many centuries to come, creating an escalating threat for coastal communities.

Sea level change through 2050 is largely locked in: Regardless of how quickly nations are able to lower emissions, the world is likely looking at about 15 to 30 centimeters (6 to 12 inches) of global average sea level rise through the middle of the century.

But beyond 2050, sea level projections become increasingly sensitive to the world’s emissions choices. If countries continue on their current paths, with greenhouse gas emissions likely to bring 3-4 C of warming (5.4-7.2 F) by 2100, the planet will be looking at a most likely sea level rise of about 0.7 meters (a bit over 2 feet). A 2 C (3.6 F) warmer world, consistent with the Paris Agreement, would see lower sea level rise, most likely about half a meter (about 1.6 feet) by 2100.

Line charts showing sea level rise accelerating the most in higher-impact scenarios.
The IPCC’s projections for global average sea level rise in meters with higher-impact pathways and the level of greenhouse gas emissions.
IPCC Sixth Assessment Report

What’s more, the more the world limits its greenhouse gas emissions, the lower the chance of triggering instabilities in the polar ice sheets that are challenging to model but could substantially increase sea level rise.

Under the most extreme emissions scenario we considered, we could not rule out rapid ice sheet loss leading to sea level rise approaching 2 meters (7 feet) by the end of this century.

Fortunately, if the world limits warming to well below 2 C, it should take many centuries for sea level rise to exceed 2 meters – a far more manageable situation. Read more