IPCC Climate Report: Profound Changes Are Underway in Earth’s Oceans and Ice – a Lead Author Explains What the Warnings Mean
ROBERT KOPP / 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.
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.
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.
Are the oceans or ice nearing any tipping points?
“Tipping point” is a vague term used in many different ways by different people. The IPCC defines tipping points as “critical thresholds beyond which a system reorganizes, in a way that is very fast or irreversible” – for example, a temperature rise beyond which climate dynamics commit an ice sheet to massive loss.
Because the term is so vague, the IPCC generally focuses on characteristics of changes in a system – for example, whether a system might change abruptly or irreversibly – rather than whether it fits the strict dynamic definition of a “tipping point.”
One example of a system that might undergo abrupt changes is the large-scale pattern of ocean circulation known as the Atlantic Meridional Overturning Circulation, or AMOC, of which the Gulf Stream is part. Paleoclimate evidence tells us that AMOC has changed rapidly in the past, and we expect that AMOC will weaken over this century. If AMOC were to collapse, it would make Europe warm more slowly, increase sea level rise along the U.S. Atlantic coast, and shift storm tracks and monsoons. However, most evidence indicates that such a collapse will not happen in this century.
Robert Kopp is a professor in the Department of Earth & Planetary Sciences at Rutgers University and director of the Rutgers Institute of Earth, Ocean, and Atmospheric Sciences. He is a lead author of the Working Group 1 contribution to the Intergovernmental Panel on Climate Change’s Sixth Assessment Report.