Warmer climate = stronger El Niño? Tree rings say maybe.

How can we learn about ancient climates? Tree rings have been a valuable clue, along with ocean sediments, chemical analysis of slow-growing corals and other record keepers of the natural world.

University of Hawai`i scientists have now found that they can track El Niño events in the rings of trees of the North American Southwest. And they've found that El Niño cycles tend to be stronger during warmer climates.

They are careful not to outright predict that global warming will mean stronger El Niño and La Niña events, but they said the data they found can be valuable in fine-tuning climate models.

El Niño, and its counterpart La Niña, involve large-scale movements of heat within the Pacific, which have related impacts on global climate. The warm phase, El Niño, features a tongue of warm water that moves eastward into the equatorial Pacific, while cooler water represents La Niña and the climate “cool” phase. Associated with them are major changes in rainfall patterns, storms and other weather features. For Hawai`i, El Niño can mean winter drought an increased hurricane activity.

One question: what will happen to El Niño patterns as the global climate warms. And one problem with that question is how to gather the information needed to understand the issue. El Niño has been around for centuries, but scientists' instruments have only been keeping track of climate for decades.

Now, researchers have found that they can track El Niño events in the tree-ring records of the U.S. Southwest over a period of 1100 years.

Meteorologists Jinbao Li and Shang-Ping Xie of the International Pacific Research Center, published their findings in the May 6 issue of Nature Climate Change.

El Niño events, with their wetter winters in the Southwest, create wider tree rings. Cold La Niña conditions promote drought there, and correspondingly narrower tree rings.

To confirm the findings, the team was able to compare the tree ring data with modern instrument data, as well as isotope concentrations in both living and ancient corals from Palmyra Atoll.

“Our work revealed that the towering trees on the mountain slopes of the U.S. Southwest and the colorful corals in the tropical Pacific both listen to the music of El Niño, which shows its signature in their yearly growth rings. The coral records, however, are brief, whereas the tree-ring records from North America supply us with a continuous El Niño record reaching back 1,100 years,” Li said.

Their findings: “During warm phases, El Niño and La Niña events were more intense than usual. During cool phases, they deviated little from the long-term average as, for instance, during the Medieval Climate Anomaly when the eastern tropical Pacific was cool,” said a press release

“Since El Niño causes climate extremes around the world, it is important to know how it will change with global warming,” Xie said. “Current models diverge in their projections of its future behavior, with some showing an increase in amplitude, some no change, and some even a decrease. Our tree-ring data offer key observational benchmarks for evaluating and perfecting climate models and their predictions of the El Niño-Southern Oscillation under global warming.”

This research was funded by the National Science Foundation, National Oceanic and Atmospheric Administration, Japan Agency for Marine-Earth Science and Technology, National Basic Research Program of China, and the National Natural Science Foundation of China.

© Jan TenBruggencate 2010

Hawaiian land crabs: Their extinction caused "profound consequences"

Hawai`i is the only tropical island group without land crabs.

Those crabs are a classic part of the environment of other warm-climate islands in Polynesia and elsewhere. It’s just another way in which these islands are unique.

But it wasn’t always so. Archaeological digs now show that land crabs once existed here, were quite common, and disappeared shortly after humans arrived—part of that first wave of extinctions that marked the changing Hawaiian environment over the past millennium or two.

The new findings were published under the title, Evolution, Insular Restriction, and Extinction of Oceanic Land Crabs, Exemplified by the Loss of an Endemic Geograpsus in the Hawaiian Islands, by Gustav Paulay of the Florida Museum of Natural History and John Starmer of the University of Florida at Gainesville. Their paper was published in the online scientific journal PLoS ONE, and can be found here.

If we ponder how much today’s Hawai`i differs from that discovered by the first Polynesians, this is an interesting window through which we can peer.

The land crabs may have been part of an entire interconnected web of life that has disappeared in the Islands. They probably were predators, feeding on the eggs of birds whose rookeries are now largely gone, on land snails that are now mostly gone, and on native insects. They had beneficial roles in the environment, helping cycle nutrients in the ground litter, spreading seeds and other things.

The Hawaiian land crab Geograpsus severnsii was once common and found on multiple islands. Its shells have been recovered from Maui and Big Island caves, Moloka`i and Kaua`i sand dunes, from the Barbers Point area of O`ahu, and elsewhere.

These were good-sized crabs, with backs 2.5 inches across, and with huge claws. While, like all crabs they retained an oceanic phase of life during their larval stages, once these crabs returned to land, they were truly terrestrial, sometimes ranging a mile or more inland, and to elevations of more than 3,000 feet.

They grew larger than any other crab in the Geograpsus genus, extended farther inland, and traveled to higher elevations.

The crabs disappear from the fossil record shortly after the arrival of humans in the islands. And their disappearance represents the world’s first extinction of a crab species in the modern age.

“That the first documented crab extinction is of a land crab in the Hawaiian Islands is not surprising. The Hawaiian terrestrial biota suffered a veritable mass extinction following human arrival, and this mass extinction continues today,” the authors write.

Rats, pigs, dogs and perhaps even human predation may have played a role in their extinction.

Their loss was beneficial, certainly, to some prey species, and harmful to other parts of the environment. In the paper abstract, the authors summarize: “Land crabs are major predators of nesting sea birds, invertebrates and plants, affect seed dispersal, control litter decomposition, and are important in nutrient cycling; their removal can lead to large-scale shifts in ecological communities. Although the importance of land crabs is obvious on remote and relatively undisturbed islands, it is less apparent on others, likely because they are decimated by humans and introduced biota. The loss of Geograpsus and potentially other land crabs likely had profound consequences for Hawaiian ecosystems.”

The authors of the study worked with fossil material collected by several scientists who worked across the state. They thanked collectors Mike Severns (for whom the species was named), David Burney, Cory Pittman, Storrs Olson, and Helen James.

© Jan TenBruggencate 2010

Citation:

^ Gustav Paulay & John Starmer (2011). "Evolution, insular restriction, and extinction of oceanic land crabs, exemplified by the loss of an endemic Geograpsus in the Hawaiian Islands". PLoS ONE 6 (5): e19916. doi:10.1371/journal.pone.0019916.