Friday, September 19, 2014

New paper: Dubious claims about California ocean habitat derived from tree-rings

Headlines today proclaim "Nearly 600 Years of Tree Rings Show Altered Ocean Habitat" due to an alleged weakening of the California Pacific Ocean coastal upwelling current, based on a paper published in Science.

Examination of the assumptions and data from the paper, however, illustrates how alarming claims in the press can be manufactured from very little scientific evidence. 

Questionable claims include:

1. Assuming the California Pacific Ocean upwelling current strength and variability are closely correlated to tree-rings, which is the proxy used, rather than ocean sediments or an actual ocean proxy.

2. Claiming [in article below the abstract] that the California Pacific Ocean upwelling current strength has weakened in the latter half of the 20th century, while the data below shows no such trend in the tree-ring proxy.

3. Claiming that the California Pacific Ocean upwelling current strength has become more variable because the moisy tree-ring proxy record has a few extra one-year-long dips as indicated by red arrows at the top of the graph below. 

a) these could simply be due to random variation in the very noisy record

b) tree-rings can be related to many factors other than precipitation and temperature, including cloud cover/cosmic rays, solar activity, CO2 plant food levels, ocean & atmospheric oscillations, etc., thus the alleged "increased variability" may not be related to upwelling current changes. 

c) the authors find these dips correlated to El Ninos, which were not exceptionally strong or variable during the latter 20th century, and have become less frequent and weaker since the beginning of the 21st century.

Thus, the paper is based upon multiple questionable assumptions that do not warrant the claims of an alarming trend in marine productivity due to an alleged weakening of coastal upwelling along the California coast.

Tree-ring data from the paper

Editor's Summary:

Rings of ocean upwelling

Coastal upwelling along the coast of California has become more variable than during nearly any period in the past 600 years. Black et al. used a 576-year tree ring record to construct a record of wintertime climate along the California coast. Because wintertime climate depends heavily on coastal upwelling, they were able to determine that upwelling variability has increased more over the past 60 years than for all but two intervals during that time. The apparent causes of the recent trend appear to be unique, resulting in reduced marine productivity and negative impacts on fish, seabirds, and mammals.

Science 19 September 2014:
Vol. 345 no. 6203 pp. 1498-1502
DOI: 10.1126/science.1253209

Six centuries of variability and extremes in a coupled marine-terrestrial ecosystem

Bryan A. Black, et al

Reported trends in the mean and variability of coastal upwelling in eastern boundary currents have raised concerns about the future of these highly productive and biodiverse marine ecosystems. However, the instrumental records on which these estimates are based are insufficiently long to determine whether such trends exceed preindustrial limits. In the California Current, a 576-year reconstruction of climate variables associated with winter upwelling indicates that variability increased over the latter 20th century to levels equaled only twice during the past 600 years. This modern trend in variance may be unique, because it appears to be driven by an unprecedented succession of extreme, downwelling-favorable, winter climate conditions that profoundly reduce productivity for marine predators of commercial and conservation interest.

Nearly 600 Years of Tree Rings Show Altered Ocean Habitat

By Kelly Dickerson, Staff Writer | September 18, 2014 02:35pm ET

Ocean currents that deliver important nutrients to shallow, coastal waters have become weaker and more variable over the last half-century, which could affect fish and other marine animals that nourish themselves in these nutrient-rich waters, according to a new study.

Data records spanning almost 600 years have shown that the strength of coastal upwelling off the west coast of North America has become more variable since 1950. Researchers pieced together this long-term look at ocean trends from an unlikely source: tree rings.

Coastal upwelling happens when winter winds lift deep, nutrient-rich waters up to the shallow layers of the sea. These nutrients fuel phytoplankton growth in the sunlit surface waters. Since 1950, California has experienced more winters with weak coastal upwelling than in the last five centuries. Researchers found that years with weak upwelling were associated with slower growth in fish populations and lower reproduction rates for seabirds, the researchers said.

But the weather pattern that causes the coastal upwelling also blocks storms from coming ashore. This causes drought and stunts the growth of trees. Blue oak trees along the California coast are particularly sensitive to winter precipitation, Bryan Black, assistant professor of marine science at the University of Texas at Austin, told Live Science.

Trees grow a new ring every year. By looking at a cross-section cut through the bark of a tree, scientists can count up the rings and determine a tree's age. Differences in the ring sizes reveal good seasons and bad seasons, with a thick ring signaling that the tree had a good growing season. The researchers found an inverse relationship between tree growth and the well-being of the marine ecosystem, Black explained.

"The winters we see robust growth in the trees, we see poor growth in the marine ecosystem," Black said.

Coastal upwelling happens during the winter when a strong, high-pressure weather system develops along the west coast of the continent. The system spins clockwise and brings in winds from the north. That spin combines with the rotation of the Earth to move the waters off shore and stir up clouds of nutrients. Phytoplankton at the surface rely on this seasonal influx of nutrients. These organisms are the backbone of the marine ecosystem and support huge populations of fish and seabirds.

Some variation in coastal upwelling from year to year is normal, but most direct data records don't go back more than 70 years. This makes it difficult for marine scientists to spot any long-term trends. By studying tree-ring patterns, however, researchers can piece together a much longer record of how coastal upwelling has changed.
To determine how upwelling influenced marine life, the researchers used data on yearly fish population growth since the 1940s, along with data on seabird egg laying and the survival of baby seabirds since the 1970s. By comparing the tree-ring data to the fish and seabird statistics, the researchers found that years with weak upwelling and lots of tree growth correlated with years when fish and seabird populations suffered.

Based on tree ring measurements taken by David Stahle, a tree ring expert and professor of geoscience at the University of Arkansas in Fayetteville, the team found that four out of the 10 weakest upwelling years in the past 600 years occurred after 1950. Seven out of 10 weakest years have happened since 1850 [the end of the Little Ice Age].

While the data show there are years in which bird and fish populations don't fare well, "it's not necessarily indicative of a long-term decline," Black said, since the bird and fish populations usually bounce back within a couple years after a bad season.
Black said it's unclear if climate change is causing the recent high variation in coastal upwelling.

"California climate can be very extreme," Black said. "The 20th century is particularly variable in the context of the last few centuries, but it's not necessarily unique to history."

The upwelling does appear to be linked to the weather pattern El Niño, and climate records have shown El Niño to be unusually variable over the past century. [not according to many other papers - El Nino's were much more intense & variable in the past]. Black said the area has certainly entered a highly variable time, but even a 600-year data record doesn't come close to capturing the whole picture. The recent variation could be part of a larger cycle that scientists can look back far enough to see.

The researchers hope to use [falsified] climate models to predict future variability in coastal upwelling. Details of the study were published online today (Sept. 18) in the journal Science.

1 comment:

  1. And there is no discussion on the fact that the Colorado River no longer flows into the Pacific Ocean?????