natural variability

Hurricane Sandy in perspective

As I write, Hurricane Sandy is making its way through New Jersey and New York wreaking havoc. Some of the press is claiming this “Frankenstorm” is a result of global warming. Some proponents of anthropogenic global warming (AGW) claim that hurricanes will become either more frequent or more intense as the planet warms.

Let’s look at some real data from Dr. Ryan Maue (http://policlimate.com/tropical/ ).

First, we see that hurricane frequency is not increasing:

Maue-Hurricane-frequency

Next, we drop down the energy scale a bit to include tropical storms and see that they, as well as hurricanes are not becoming more frequent.

Maue-tropical-storm-and-hurricane-frequency

Finally, we look at storm intensity and see that although this is quite variable, there is no overall trend:

Maue-tropical-cyclone-energy

From these data we see that hurricanes fail to follow the AGW predictions. So, either AGW alarmists are wrong or there is no recent global warming.

Is Sandy unprecedented? No, but it is unusually strong and widespread because of the confluence of a hurricane with a major cold front from the north.

Sandy made landfall in New Jersey, so look at the history of such hurricanes from “Hurricanes and the Middle Atlantic States.” That source provides brief descriptions of 21 hurricanes that affected New Jersey between 1821 and 2011. That list includes Hurricane Hazel on October 15, 1954. So even October hurricanes are not unprecedented.

Dr. Judith Curry, in a very long assessment of lessons learned since Hurricane Katrina in 2005, notes that there is still uncertainty of exact causes of frequency and intensity. But, the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO) have dominant impacts on hurricane variability in the Pacific, and modulates the frequencies of El Nino/La Nina. Curry says, “I suspect that the combination of the PDO and NPGO can explain much of the variability in Ryan Maue’s analysis of Accumulated Cyclone Energy diagram, given that the majority of global hurricanes occur in the Pacific.” “In the Atlantic, the Atlantic Multidecadal Oscillation (AMO), the North Atlantic Oscillation (NAO), and Atlantic Meridional Mode (AMM) have all been invoked to explain variability in the Atlantic …”

Although the science of exact attribution is not settled, it does seem that AGW is not the answer since natural variability provides an explanation.

See also:

IPCC says they don’t know if the climate is becoming more extreme

Weather extremes and global warming – no increasing trend

IPCC says climate change signals small compared to natural climate variability

Some real science might have leaked from IPCC

British reporter Richard Black of the BBC claims to have received a draft of the Intergovernmental Panel on Climate Change (IPCC) report on extreme weather.

 In this draft, the IPCC seems to have backed off some of its past scary predictions and now says in the report:

 There is “low confidence” that tropical cyclones have become more frequent, “limited-to-medium evidence available” to assess whether climatic factors have changed the frequency of floods, and “low confidence” on a global scale even on whether the frequency has risen or fallen.

 In terms of attribution of trends to rising greenhouse gas concentrations, the uncertainties continue.

 While it is “likely” that anthropogenic influences are behind the changes in cold days and warm days, there is only “medium confidence” that they are behind changes in extreme rainfall events, and “low confidence” in attributing any changes in tropical cyclone activity to greenhouse gas emissions or anything else humanity has done.

 “Uncertainty in the sign of projected changes in climate extremes over the coming two to three decades is relatively large because climate change signals are expected to be relatively small compared to natural climate variability”.

 In that last paragraph, the IPCC admits that they don’t know what will happen.  This draft report must be vetted by policy makers and a final report is scheduled to be released Friday.  We will see how many of these “uncertainty” statements survive.

 See some other opinions/reports on this here, here, and here.

See also:

A Perspective on Climate Change a tutorial

Carbon Dioxide and the Greenhouse Effect

Humans and the Carbon Cycle

African Lake Study Leaves Some Questions

The headline from the University of Arizona News, and many other news outlets said, “Twentieth-Century Warming in Lake Tanganyika is Unprecedented.” The headline from Brown University press release (home of the lead author) said, “Brown Geologists Show Unprecedented Warming in Lake Tanganyika.”

Well, not exactly. The title of the study referred to is “Late-twentieth-century warming in Lake Tanganyika unprecedented since AD 500,” published in Nature Geoscience (16 May 2010). Even that more modest claim doesn’t tell the whole story.

First some background. Lake Tanganyika occurs within the East African Rift, which is a divergent tectonic plate boundary that is gradually separating East African countries from the main continent. The rift contains both active and dormant volcanoes. The lake is 418 miles long and 45 miles wide. Its average depth is 1,870 feet with a maximum depth of 4,820 feet. Portions of the lake are claimed by Burundi, Democratic Republic of the Congo, Tanzania, and Zambia. Fishing the lake provides a major food source for people in the surrounding lands. There is concern that lake warming will disrupt the fish supply.

The abstract of the paper concludes, “Our records indicate that changes in the temperature of Lake Tanganyika in the past few decades exceed previous natural variability. We conclude that these unprecedented temperatures and a corresponding decrease in productivity can be attributed to anthropogenic global warming, with potentially important implications for the Lake Tanganyika fishery.”

The questions I had upon reading this were: 1) Are the temperatures really unprecedented? 2) Do they exceed natural variability? 3) What is the evidence that the warming was caused by anthropogenic global warming? 4) Could there be some other cause of fish decline?

The researchers studied lake sediment cores going back 60,000 years and by using proxies deduced a temperature record for the lake surface temperature. In the current study, the researchers said that during the last 1,500 years, temperature varied between 22.5º C and 25.7º C, and that in the last 50 years the temperature rose by 1.6º C.

However, in 2008, these same researchers published a paper in Science (Vol. 322. no. 5899, pp. 252 – 255) which said the lake surface temperature fluctuated between 27° and 29°C over the last 60,000 years according to their interpretation of lake sediment cores.

I emailed a co-author of the paper, a UofA professor, asking for an explanation of this apparent discrepancy. He replied by referring me to the website of the lead author at Brown University. There, she explained that there was a problem in calibration of the temperature proxies. She presents a graph showing the records after recalibration. It is reproduced below. It should be noted that there are two separate core sample locations. The more recent core was taken closer to shore than the older, longer record. The more recent record initially shows cooler temperatures where the two records overlap. The researchers attribute this discrepancy to upwelling cold water from deeper in the lake. So which record is closer to the real surface temperature?

TanganyikaTemp

According to the lead author’s own data as shown on the graph, it is obvious that the current temperatures are not unprecedented, nor do they exceed natural variability. The title of their paper is technically correct only if one accepts cherry-picking start dates.

That leaves the question about the cause of the warming. The UofA scientist replied to my email, “our record only demonstrates a lake surface temperature history, not the cause of that history.” The allegation of an anthropogenic cause, a major conclusion of the paper, was made without any supporting evidence, just speculation.

I am wondering why the paper abstract contains the conclusions it does. Is it time for some scary scenarios to promote more study and more funding?

This whole study purports to be about lake surface temperatures, but it contains very few such measurements from the lake surface. From my reading, the researchers deduce surface temperatures from only two core sample locations. As the NOAA satellite graphic below shows, on any given day, at any given time, the variation in lake surface temperature can be as much as 4º C in different parts of the lake, and that equals or exceeds the entire range of temperatures found in the studies. It would seem, therefore, that any temperature record derived from sediment cores could vary greatly depending on location. Since this study had just two sample locations, it makes one wonder if it gives a true representation of actual conditions.

satellite_tanganyika

And about the fish. The current paper says that warming is causing a decline in fish abundance. Yet an earlier study, of which the UofA scientist was a co-author, says the fish decline is caused by land disturbance. “Watershed deforestation, road building, and other anthropogenic activities result in sediment inundation of lacustrine habitats.” “Our faunal analyses suggest that all three taxonomic groups are negatively affected by sediment inundation but may have varying response thresholds to disturbance.” (Citation: Conservation Biology, vol. 13, no. 5, Oct. 1999).