Posted by Antony Green on October 03, 2013 at 11:34 AM | Permalink

From Dr. Josh Willis, NASA/Jet Propulsion Laboratory oceanographer

This famous climate scientist I know used to always begin his talks on sea level rise by showing a cartoon of a bathtub. A faucet would fill up the bathtub, representing water coming from melting glaciers and ice sheets, and then a small campfire would add heat, causing the water to expand. It was a nice way of getting people to think about the causes of sea level rise. But you might guess that the real ocean is a bit more complicated than the cartoon lets on. And you’d be right. Even though it’s sometimes convenient to think of the ocean as a great big bathtub, where turning on the tap at one end raises the water level in the whole tub, real sea level rise doesn’t quite happen that way. To understand why, you first have to realize that ‘sea level’ isn’t really level at all.

There are lots of reasons why the oceans are not level. For example, vast ocean currents like the Gulf Stream in the Atlantic Ocean and the Kuroshio in the Pacific actually reshape the ocean surface, causing it to tilt. As the planet heats up, changes in the prevailing winds (which drive most of these ocean currents) cause changes in the currents, reshaping our ocean and changing local sea level as a result.

Just as global warming does not raise land temperatures evenly, global ocean warming is not the same everywhere around the globe. Some regions of the oceans are heating up faster than others, and because warm water takes up more space than cold water, those regions experience faster sea level rise.

Finally, the water locked away in the great ice sheets of Greenland and Antarctica also shapes the ocean surface. As the ice sheets melt and lose water to the oceans, our entire planet feels the effects. The movement of mass from the ice sheets to the oceans very slightly shifts the direction of Earth’s rotation. This, along with changes in the gravitational pull of the ice sheets on the oceans, will reshape sea levels further still.

So sea level rise is complicated, and the bathtub analogy just doesn’t cut it. Given all that, the image below is not that surprising. It shows how sea levels changed between 1993 and 2008 (yellow and red show rising sea levels, blue and green show falling). Since the early 1990s, space satellites have given us a bird’s-eye view of sea level change. They tell us that that sea level, averaged over the whole planet, has gone up by about 4.5 cm (1.8 inches) since 1993 — clear evidence of global warming. (To recap: global warming causes a) ocean water to warm up, expand, and take up more space; and b) glaciers to melt which add more water to the oceans.) But the patterns of sea level change tell a much more complicated story.

Most of the regional variations seen over this 16-year trend are related to changes in the ocean currents rather than the movement or melting of ice. Australia for example, where 80% of the population lives near the coast, has seen sea levels rise much more quickly than the rest of the planet. Meanwhile, along the California coast, sea level has stayed steady or even fallen a bit.

The question is whether these regional differences are part of natural climate cycles, or an effect of global warming? The answer is probably both, and scientists around the world are working hard to tell the difference.

So where does all this leave my friend’s bathtub analogy? High and dry, of course.

The change in the world’s sea level between 1993 and 2008. Black areas are land; colored are the oceans. Yellow and red regions show rising sea level, while green and blue regions show falling sea level. White regions are missing data during parts of the year. Nearly everywhere, sea level is rising, and the global average is clearly rising fast. But the patterns of sea level change (the regional variations) are complicated.

Credit: Altimeter data products include data from TOPEX/Poseidon, Jason-1 and Jason-2 and other satellites. Data products come from Ssalto/Duacs, distributed by Aviso, with support from CNES.

Editor’s note: If you’re interested in how sea level is changing around the globe check out our Climate Time Machine and Sea Level Viewer.

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Australia’s warmest 12-month period on record, again

Australia’s warmest September on record

Australia’s record for warmest 12-month period has been broken for a second consecutive month. This continues a remarkable sequence of warmer-than-average months for Australia since June 2012.

September 2013 was easily Australia’s warmest September on record. The national average temperature for September was +2.75 °C above the long-term (1961–1990) average, which also sets a record for Australia’s largest positive anomaly for any monthly mean temperature. The previous record of +2.66 °C was set in April 2005.

The mean temperature for Australia, averaged over the 12 months from October 2012 to September 2013, was 1.25 °C above the long-term average. This was also 0.17 °C warmer than any 12-month period prior to 2013.

The previous record, set over September 2012 to August 2013, was +1.11 °C above the long-term average, and the record preceding the current warm spell was +1.08 °C, set between February 2005 and January 2006.

Temperatures for the calendar year to date (January to September) have also been the warmest on record, at 1.31 °C above the long-term average, well above the figure set for January to September 2005 (+1.07 °C). 2005 currently holds the record for Australia’s warmest calendar year.

The past 18 months have been characterised by widespread heat across Australia. The mean temperature has been above average over the entire continent.

October 2012 to September 2013 mean temperature deciles for Australia showing 12-month temperatures were the highest on record for 39% of Australia.

In the past 12-month period a large number of mean temperature records have fallen across Australia including:

• Australia’s warmest month on record (January)
• Australia’s warmest September on record
• Australia’s largest positive monthly anomaly on record (September)
• Australia’s warmest summer on record (December 2012 to February 2013)
• Australia’s warmest January to September period on record
• Australia’s warmest 12-month period on record (broken twice, for the periods ending August and September)
• Indeed, Australia’s warmest period on record for all periods 1 to 18 months long ending September 2013

Two significant daily maximum temperature records were also set this year:

• Australia’s hottest summer day on record (7 January)
• Australia’s warmest winter day on record (31 August)

The periods inclusive of September 2013 have also resulted in numerous State and Territory mean temperature records including:

• Warmest September on record for South Australia, Victoria, New South Wales, Queensland and the Northern Territory
• Largest positive monthly anomaly on record for South Australia and Queensland (September)
• The warmest January to September period on record for South Australia, Victoria, New South Wales and the Northern Territory, and also for Melbourne, Sydney and Adelaide
• The warmest 12-month period on record for South Australia, the Northern Territory, and southern Australia

In addition to these records, and those set during the heat events of January and autumn, many individual stations have set records for early season heat or September record highs.

Generally above-average temperatures have persisted with few breaks since September 2012. The period has been characterised by long periods of warmer-than-average days and a distinct lack of cold weather. Nights have also been warmer than average, but less so than daytime temperatures.

Every calendar month since September 2012 has recorded temperatures at least 0.5 °C above average, with eight of those thirteen months topping 1.0 °C above average including January, April, May, July, August and September of 2013. Widespread record warmth has also been recorded in the oceans around Australia.

Monthly mean temperature anomalies for September 2012 to September 2013.

The Remainder of 2013

The year-to-date temperature anomaly is currently so large that mean temperatures during the remaining three months of 2013 only need to be slightly above average for the year to set a new calendar year record. An anomaly of just +0.24 °C for the remainder of the year (i.e. October to December) will result in 2013 equalling the record held by 2005. Zero anomalies for the last three months of the year will result in 2013 finishing as the second-warmest year on record.

It also remains possible that the 12-month mean temperature record will be equalled or broken again before the end of the year, for the periods ending October, November and/or December.

The Bureau’s latest seasonal outlook indicates that both maximum and minimum temperatures are most likely to be above average over most of Australia. The outlook for warmer-than-average temperatures over the remainder of the year is consistent with the Bureau’s knowledge of background temperature trends, and the well-above-average sea surface temperatures that currently surround Australia. Australian temperatures are now on average more than 1 °C warmer than during the 1950s.

Running means for Australian temperature anomalies for 12-month periods ending September 2013. Vertical grid lines mark 12-month periods commencing January 1920, January 1930, etc.