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Does global warming make tropical cyclones stronger?

Filed under: — stefan @ 30 May 2018

By Stefan Rahmstorf, Kerry Emanuel, Mike Mann and Jim Kossin

Friday marks the official start of the Atlantic hurricane season, which will be watched with interest after last year’s season broke a number of records and e.g. devastated Puerto Rico’s power grid, causing serious problems that persist today. One of us (Mike) is part of a team that has issued a seasonal forecast (see Kozar et al 2012) calling for a roughly average season in terms of overall activity (10 +/- 3 named storms), with tropical Atlantic warmth constituting a favorable factor, but predicted El Nino conditions an unfavorable factor.  Meanwhile, the first named storm, Alberto, has gone ahead without waiting for the official start of the season.

In the long term, whether we will see fewer or more tropical cyclones in the Atlantic or in other basins as a consequence of anthropogenic climate change is still much-debated. There is a mounting consensus, however, that we will see more intense hurricanes. So let us revisit the question of whether global warming is leading to more intense tropical storms. Let’s take a step back and look at this issue globally, not just for the Atlantic. More »

If you doubt that the AMOC has weakened, read this

A few weeks ago, we’ve argued in a paper in Nature that the Atlantic overturning circulation (sometimes popularly dubbed the Gulf Stream System) has weakened significantly since the late 19th Century, with most of the decline happening since the mid-20th Century. We have since received much praise for our study from colleagues around the world (thanks for that). But there were also some questions and criticisms in the media, so I’d like to present a forum here for discussing these questions and hope that others (particularly those with a different view) will weigh in in the comments section below. More »

Stronger evidence for a weaker Atlantic overturning circulation

Filed under: — stefan @ 11 April 2018

Through two new studies in Nature, the weakening of the Gulf Stream System is back in the scientific headlines. But even before that, interesting new papers have been published – high time for an update on this topic.

Let’s start with tomorrow’s issue of Nature, which besides the two new studies (one of which I was involved in) also includes a News&Views commentary. Everything revolves around the question of whether the Gulf Stream System has already weakened. Climate models predict this will be one consequence of global warming – alongside other problems such as rising sea levels and increasing heat waves, droughts and extreme precipitation. But is such a slowdown already underway today? This question is easier asked than answered. The Atlantic Meridional Overturning Circulation (AMOC, also known as Gulf Stream System) is a huge, three-dimensional flow system throughout the Atlantic, which fluctuates on different time scales. It is therefore by no means enough to put a current meter in the water at one or two points. More »

The global CO2 rise: the facts, Exxon and the favorite denial tricks

Filed under: — stefan @ 25 January 2018

The basic facts about the global increase of CO2 in our atmosphere are clear and established beyond reasonable doubt. Nevertheless, I’ve recently seen some of the old myths peddled by “climate skeptics” pop up again. Are the forests responsible for the CO2 increase? Or volcanoes? Or perhaps the oceans?

Let’s start with a brief overview of the most important data and facts about the increase in the carbon dioxide concentration in the atmosphere:

  1. Since the beginning of industrialization, the CO2 concentration has risen from 280 ppm (the value of the previous millennia of the Holocene) to now 405 ppm.
  2. This increase by 45 percent (or 125 ppm) is completely caused by humans.
  3. The CO2 concentration is thus now already higher than it has been for several million years.
  4. The additional 125 ppm CO2 have a heating effect of 2 watts per square meter of earth surface, due to the well-known greenhouse effect – enough to raise the global temperature by around 1°C until the present.

Fig. 1 Perhaps the most important scientific measurement series of the 20th century: the CO2 concentration of the atmosphere, measured on Mauna Loa in Hawaii. Other stations of the global CO2 measurement network show almost exactly the same; the most important regional variation is the greatly subdued seasonal cycle at stations in the southern hemisphere. This seasonal variation is mainly due to the “inhaling and exhaling” of the forests over the year on the land masses of the northern hemisphere. Source (updated daily): Scripps Institution of Oceanography. More »

Snow Water Ice and Water and Adaptive Actions for a Changing Arctic

The Arctic is changing fast, and the Arctic Council recently commissioned the Arctic Monitoring and Assessment Programme (AMAP) to write two new reports on the state of the Arctic cryosphere (snow, water, and ice) and how the people and the ecosystems in the Arctic can live with these changes.

The two reports have now just been published and are called Snow Water Ice and Permafrost in the Arctic Update (SWIPA-update) and Adaptive Actions for a Changing Arctic (AACA).

More »