In the long run, sea-level rise will be one of the most serious consequences of global warming. But how fast will sea levels rise? Model simulations are still associated with considerable uncertainty – too complex and varied are the processes that contribute to the increase. A just-published survey of 90 sea-level experts from 18 countries now reveals what amount of sea-level rise the wider expert community expects. With successful, strong mitigation measures, the experts expect a likely rise of 40-60 cm in this century and 60-100 cm by the year 2300. With unmitigated warming, however, the likely range is 70-120 cm by 2100 and two to three meters by the year 2300.
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Global Warming Since 1997 Underestimated by Half
A new study by British and Canadian researchers shows that the global temperature rise of the past 15 years has been greatly underestimated. The reason is the data gaps in the weather station network, especially in the Arctic. If you fill these data gaps using satellite measurements, the warming trend is more than doubled in the widely used HadCRUT4 data, and the much-discussed “warming pause” has virtually disappeared. [Read more…] about Global Warming Since 1997 Underestimated by Half
Sea level in the 5th IPCC report
What is happening to sea levels? That was perhaps the most controversial issue in the 4th IPCC report of 2007. The new report of the Intergovernmental Panel on Climate Change is out now, and here I will discuss what IPCC has to say about sea-level rise (as I did here after the 4th report).
Let us jump straight in with the following graph which nicely sums up the key findings about past and future sea-level rise: (1) global sea level is rising, (2) this rise has accelerated since pre-industrial times and (3) it will accelerate further in this century. The projections for the future are much higher and more credible than those in the 4th report but possibly still a bit conservative, as we will discuss in more detail below. For high emissions IPCC now predicts a global rise by 52-98 cm by the year 2100, which would threaten the survival of coastal cities and entire island nations. But even with aggressive emissions reductions, a rise by 28-61 cm is predicted. Even under this highly optimistic scenario we might see over half a meter of sea-level rise, with serious impacts on many coastal areas, including coastal erosion and a greatly increased risk of flooding.
Fig. 1. Past and future sea-level rise. For the past, proxy data are shown in light purple and tide gauge data in blue. For the future, the IPCC projections for very high emissions (red, RCP8.5 scenario) and very low emissions (blue, RCP2.6 scenario) are shown. Source: IPCC AR5 Fig. 13.27.
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Climate Change on Film
Making a film about climate change is difficult, especially if you want it to reach a wide audience. One problem is the long time scale of climate change, which fits badly with the time scale of a typical film narrative. That was the reason why in the Hollywood blockbuster The Day After Tomorrow some laws of physics were treated with a certain artistic freedom, in order to present a dramatic climate change within a few weeks instead of decades.
Mike and I have spent the last few days at a very interesting workshop in Iceland, where climate scientists, social scientists and filmmakers were brought together in conjunction with the Reykjavik International Film Festival. I will make no attempt to reproduce the many exciting discussions which we had, that often continued into the night. Instead, I’d like to present two short films by workshop participants. I chose a contrast of hot and cold.
First, the cold. The following film is a trailer by Phil Coates, a British filmmaker and expedition leader, who has filmed in extreme conditions on all seven continents. It is a “work in progress” under the working title “North Pole Living on Thin Ice”. Coates was dropped off with three scientists on the sea ice near the North Pole. On foot out on the Arctic Ocean they made oceanographic and ice thickness measurements. Soon you will be able to experience this research expedition on film. The scientific findings of the team will of course come out in the scientific literature.
http://vimeo.com/70558290#
The new IPCC climate report
The time has come: the new IPCC report is here! After several years of work by over 800 scientists from around the world, and after days of extensive discussion at the IPCC plenary meeting in Stockholm, the Summary for Policymakers was formally adopted at 5 o’clock this morning. Congratulations to all the colleagues who were there and worked night shifts. The full text of the report will be available online beginning of next week. Realclimate summarizes the key findings and shows the most interesting graphs.
Update 29 Sept: Full (un-copyedited) report available here.
Global warming
It is now considered even more certain (> 95%) that human influence has been the dominant cause of the observed warming since the mid-20th century. Natural internal variability and natural external forcings (eg the sun) have contributed virtually nothing to the warming since 1950 – the share of these factors was narrowed down by IPCC to ± 0.1 degrees. The measured temperature evolution is shown in the following graph.
Figure 1 The measured global temperature curve from several data sets. Top: annual values. Bottom: averaged values over a decade.
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What ocean heating reveals about global warming
The heat content of the oceans is growing and growing. That means that the greenhouse effect has not taken a pause and the cold sun is not noticeably slowing global warming.
NOAA posts regularly updated measurements of the amount of heat stored in the bulk of the oceans. For the upper 2000 m (deeper than that not much happens) it looks like this:
Change in the heat content in the upper 2000 m of the world’s oceans. Source: NOAA
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Paleoclimate: The End of the Holocene
Recently a group of researchers from Harvard and Oregon State University has published the first global temperature reconstruction for the last 11,000 years – that’s the whole Holocene (Marcott et al. 2013). The results are striking and worthy of further discussion, after the authors have already commented on their results in this blog.
The inevitability of sea level rise
Guest post by Anders Levermann [via The Conversation]
Small numbers can imply big things. Global sea level rose by a little less than 0.2 metres during the 20th century – mainly in response to the 0.8 °C of warming humans have caused through greenhouse gas emissions. That might not look like something to worry about. But there is no doubt that for the next century, sea level will continue to rise substantially. The multi-billion-dollar question is: by how much? [Read more…] about The inevitability of sea level rise
Sea-level rise: Where we stand at the start of 2013 — Part 2
This is Part 2 of my thoughts on the state of sea-level research. Here is Part 1.
Sea-level cycles?
A topic that keeps coming up in the literature is the discussion on a (roughly) 60-year cycle in sea level data; a nice recent paper on this is Chambers et al. in GRL (2012). One thing I like about this paper is its careful discussion of the sampling issue of the tide gauges, which means that variability in the tide gauges is not necessarily variability in the true global mean sea level (see Part 1 of this post). I want to add some thoughts on the interpretation of this variability. Consider this graph from my Response to Comments in Science (2007):
Fig. 1: Fifteen-year averages of the global mean temperature (blue, °C, GISS data) and rate of sea level rise (red, cm/year, Church&white data), both detrended.
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References
- D.P. Chambers, M.A. Merrifield, and R.S. Nerem, "Is there a 60‐year oscillation in global mean sea level?", Geophysical Research Letters, vol. 39, 2012. http://dx.doi.org/10.1029/2012GL052885
- S. Rahmstorf, "Response to Comments on "A Semi-Empirical Approach to Projecting Future Sea-Level Rise"", Science, vol. 317, pp. 1866-1866, 2007. http://dx.doi.org/10.1126/science.1141283
Sea-level rise: Where we stand at the start of 2013
Progress has been made in recent years in understanding the observed past sea-level rise. As a result, process-based projections of future sea-level rise have become dramatically higher and are now closer to semi-empirical projections. However, process-based models still underestimate past sea-level rise, and they still project a smaller rise than semi-empirical models.
Sea-level projections were probably the most controversial aspect of the 4th IPCC report, published in 2007. As an author of the paleoclimate chapter, I was involved in some of the sea-level discussions during preparation of the report, but I was not part of the writing team for the projections. At the core of the controversy were the IPCC-projections which are based on process models (i.e. models that aim to simulate individual processes like thermal expansion or glacier melt). Many scientists felt that these models were not mature and understated the sea-level rise to be expected in future, and the IPCC report itself documented the fact that the models seriously underestimated past sea-level rise. (See our in-depth discussion published after the 4th IPCC report appeared.) That was confirmed again with the most recent data in Rahmstorf et al. 2012.
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References
- S. Rahmstorf, G. Foster, and A. Cazenave, "Comparing climate projections to observations up to 2011", Environmental Research Letters, vol. 7, pp. 044035, 2012. http://dx.doi.org/10.1088/1748-9326/7/4/044035