Key science report shows "unprecedented" changes to oceans and frozen regions

Global warming is greatly transforming the planet's oceans and frozen regions, and future emissions levels will dictate how much additional harm unfolds this century and beyond, a major United Nations-led scientific analysis shows.

Why it matters: "The ocean is warmer, more acidic and less productive. Melting glaciers and ice sheets are causing sea level rise, and coastal extreme events are becoming more severe," the UN's Intergovernmental Panel on Climate Change (IPCC) said in a statement alongside Wednesday's report.

Why you'll hear about this again: Periodic analyses from large groups of scientists convened by the IPCC provide a benchmark for policymakers, diplomats and advocates.

• The report arrives at a time of political focus on global warming in the U.S. and more widely — attention driven by the U.S. presidential election, where Democratic hopefuls are pushing aggressive plans, and the youth-led worldwide climate movement.

The big picture: The report sizes up the stakes for over 1.3 billion people living in low-lying coastal regions or high mountain areas, as well as animal species and ecosystems.

• More broadly, oceans and the cryosphere — permafrost, sea-ice, glaciers and so forth — are "interconnected with other components of the climate system."

Where it stands: The report finds "widespread shrinking" of the cryosphere in past decades due to global warming, including:

• The Greenland Ice Sheet losing mass at an average rate of 278 billion tons per year between 2006 and 2015.
• June Arctic snow cover reach on land fell by roughly 2.5 million kilometers from 1967-2018.
• Arctic sea ice cover in September, the yearly minimum time, very likely fell around 13% per decade between 1979 and 2018. "These sea ice changes in September are likely unprecedented for at least 1000 years," it notes.

When it comes to oceans, the rate of warming has likely more than doubled since 1993. Global mean sea-level rise 2006 –2015 is very likely 3.1 –4.1 millimeters annually, which is "unprecedented over the last century."

Where it stands: The report takes stock of many harms already unfolding for humans and other species due to warming-induced changes.

• Examples include marine heatwaves causing large-scale coral bleaching; contracting habitat for certain mammals and birds due to sea-ice loss; and sea-level rise adding to a variety of coastal damages.

• Since the mid-20th century, the shrinking cryosphere in the Arctic and high mountain regions has brought "predominantly negative" effects on food security, water quality, infrastructure and more, the report finds.

What's next: Ongoing changes are unavoidable, but the ultimate extent depends on whether and by how much nations are able rein in rising greenhouse gas emissions.

• For instance, loss of glacier mass by 2100 is estimated to be roughly twice as high if emissions were to soar for decades, bringing very high warming levels, compared to a scenario in which emissions peak soon and fall sharply over the century.
• Sea-level rise will continue and could reach 30-60 centimeters by 2100 even if emissions fall sharply and temperature rise is held well below 2°C (the goal of the Paris climate agreement), but reach 60-110cm, or up to 3.6 feet, "if greenhouse gas emissions continue to increase strongly," a summary notes.

The bottom line: "If we reduce emissions sharply, consequences for people and their livelihoods will still be challenging , but potentially more manageable for those who are most vulnerable," IPCC chair Hoesung Lee said in a statement.

Go deeper: Why climate change is so hard to tackle: the global problem