February 27, 2007

Concern for Kelp Crippled

Filed under: Plants

An article has appeared in the recent issue of Global Change Biology entitled “Little evidence for climate effects on local-scale structure and dynamics of California kelp forest communities.” Is this a mistake, a joke, or some kind of hoax? Did the authors, reviewers, and editors of this outstanding journal not get the message that global warming is destroying ocean ecosystems throughout the world? Everything on land and under the sea is enormously and negatively impacted by ongoing climate change related to the buildup of greenhouse gases – right?

Think again.

If you search for “global warming and kelp” on the internet, you will find 65,000 websites with almost nothing but bad news for kelp forests thanks to the many horrors of global warming. The story told repeatedly is that global warming will cripple kelp because the underwater forest grows in areas of the world with cold water and cannot survive or reproduce in waters above 68°F. Since many kelp forests are currently growing in water temperatures between 60°F and 65°F, any warming seriously jeopardizes the fate of these underwater jungles. Secondly, you will learn repeatedly that “Altered weather patterns cause violent storms that can tear kelp loose from the bottom.” If you have read World Climate Report over the years, you will seriously question the notion that “violent storms” will increase, but if you are looking for the kelp story, the destructive storm link to global warming is everywhere. Finally, you will find statements like “Sea urchins are having a severe impact. They are normally passive grazers of algae but when ocean waters warm up, urchins graze kelp instead.” You might also encounter comments that global warming alters nutrient supplies, disrupts the balance between fish, lobsters, and urchins, causes changes in underwater currents, changes runoff of streams and rivers into the ocean, alters the natural cycle of El Niño and La Niña, and on and on. One prominent website suggests all the global warming damage to kelp forests is ultimately caused by “government inaction and procrastination” (i.e., major industrialized countries that did not sign the Kyoto Protocol).

The recent article in Global Change Biology was produced by two scientists with the National Center for Ecological Analysis and Synthesis in Santa Barbara, California and Department of Integrative Biology at the University of Guelph in Ontario. Halpern and Cottenie note that “sea surface temperature can cause entire coral reefs to bleach, in turn shifting the composition of fish and invertebrate communities, and shifts in water temperature and storm disturbance have had dramatic effects on the abundance and distribution of giant kelp.” However, that statement is specific to changes in El Niño – Southern Oscillation (ENSO) conditions, not necessarily to impacts of global changes in temperature over longer periods of time.

Halpern and Cottenie note that the “Kelp Forest Monitoring Program (KFMP), run by the US National Park Service, has surveyed the densities of many algae, invertebrates, and fish annually since 1983 at 16 different rocky reef sites around the five northern Channel Islands, CA, USA that are part of the Channel Islands National Park.” In addition to the extensive KFMP datasets, Halpern and Cottenie also collected data on local water temperatures in the Channel Islands, regional wave disturbance levels, and regional ENSO values. They generated variables such as the number of months with the sea surface temperatures above and below 61.7°F, days with wave heights above 8.8 feet, and antecedent ENSO conditions. They used very sophisticated statistical techniques to link conditions of the kelp forest to variations in local and regional climate conditions to “explicitly evaluate both the absolute and relative importance of climate on kelp forest community structure and dynamics.”

They found that the average temperature and the number of months above or below the 61.7°F threshold “explained a very small portion of the variation in community structure.” With respect to response to ENSO variability, they note that “we found little evidence for community-level responses to variation in climatic variables on kelp forest community structure.” Temperature variations explained all of two percent of the variance in kelp community structure and the wave disturbance variable explained only 0.5 percent of the variance through time. Halpern and Cottenie state “If differences in climate variables are having an effect on local kelp forest community structure, those effects are being swamped by other local-scale biological or physical variables. Given the uniquely large spatial, temporal, and taxonomic scale of the data used for these analyses, these results should be highly robust.” The further note that other researchers a decade ago “using primarily measurements of diversity, found that Southern Californian temperate reef fish communities were similarly insensitive to climatic variation.”

Halpern and Cottenie conclude that “differences in kelp forest community structure and dynamics around the Channel Islands are only marginally related to climatic variables. Consequently, future changes in local and regional scale climate may have only a small effect on kelp forest communities.”

Imagine what would have happened if the pair of kelp experts had found a statistical relationship between local and/or regional warming and negative consequences to the kelp forest aquatic ecosystem. Their faces would be on television everywhere, their work would be featured in prominent news magazines, every newspaper in the world would carry the story, and 65,000 web pages would feature the results. But from the time we read the title of their article, we suspected Halpern and Cottenie’s findings about kelp forests and global warming would get them nothing but a snub from the greenhouse crusade.


Halpern, B.S. and K. Cottenie, 2007. Little evidence for climate effects on local-scale structure and dynamics of California kelp forest communities. Global Change Biology, 13, 236–251.

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