Though oceans are warming more slowly than land on average, climate changes still put sea creatures under similar pressure to those on dry land. That’s what Michael Burrows from the Scottish Marine Institute in Oban and 18 other researchers from eight different countries found by analysing the world’s surface temperature over the past 50 years. They brought together rates and directions of temperate change into a measure called “velocity of climate change”, showing how quickly and far species would have to move to stay in a similar environment. “It so happens that there are an awful lot of species in some of the places where velocities are highest, especially in the tropical oceans,” Burrows told Simple Climate. “That may have damaging effects on the richness of species in those places, especially in the hottest places, where there can be no immigrants from even hotter places to replace those that leave.”
These findings are the outcome of a series of meetings the researchers held at the University of California, Santa Barbara to assess the evidence for ocean life responding to climate change. “It struck us right away that there were no expectations available for how far organisms should shift to track temperatures, or by how much earlier or later they should do things seasonally,” Burrows explained. “We thought we could analyse temperature data to make these predictions.” Once they decided to do the analysis, it was surprisingly easy, the ecologist said, as worldwide temperature data is already available for the 20th century and up to the present.
Putting warming in its place
Velocity of climate change is the term that describes how far organisms should shift. “Imagine a line across a map that represents all those places where the temperature is the same, like a height contour,” said Burrows. “That’s called an isotherm. If the climate warms, that line will move in the direction of cooler temperatures, and the speed and direction of that movement is the ‘velocity of climate change’. If the distributions of animals and plants move to keep them in the same temperatures, we expect that movement to follow the isotherms at the velocity of climate change.” To look at how much earlier or later organisms should do seasonal activities like mating, the researchers also looked at “seasonal climate shifts”. This is the number of days per year earlier temperatures are reached in different locations.
Writing in top research journal Science last week, the team noted the land median average rate of warming since 1960 was 0.24°C per decade. This compared against 0.07°C per decade in the sea. Yet, seasonal climate shifts were greater in the sea than on land, mainly because there is usually less of a temperature difference between seasons in the ocean, so warming has a greater effect. At latitudes where there are both land and sea, ocean velocity of climate change was 24.5 km per decade, almost exactly the same as on land. That’s in part because across most of the world’s land it’s comparatively easy to climb to higher altitudes to cool down, but sea creatures are typically used to living at a specific depth. In the sea, species therefore have to travel across large areas of similar temperature water before crossing a relatively sharp boundary into a cooler area.
Schooling to cool waters
Burrows notes that these local factors can be more important than worldwide trends. “Life may need to move fast to track even small temperature changes in some parts of the world,” he said. “In other places, larger temperature changes might need only small displacements for animals and plants to stay in their preferred temperatures. So an average rate of shift may not be that useful. In essence, regions where we show the fastest velocities may be most prone to the loss of species and potentially gaining new ones, while where velocities are slower there may be some movement of the boundaries of species, fishes ranging further north for example, but probably keeping the same mix for a much longer time.”
With regions rich in marine biodiversity often having high velocities of climate change and strong seasonal shifts, there’s a big question about how humans can help species in those areas. “Conservation measures can’t usually protect species from direct effects of climate change,” Burrows noted, although these findings do provide some ideas of what will be helpful. “We should especially protect those areas which provide longer term refuges, where velocities are low, and where refugees from climate change may arrive,” the scientist said. “This suggests that areas of slower velocities immediately outside the tropics are worthy of special measures. Coral reefs may need new areas to colonise if they are not to go extinct.”
Having only looked at temperature records so far, Burrows and his team now want to compare observed biological changes with their calculated velocity of climate change and seasonal shift. They also want to exploit the work to help conservation efforts. “We’d like to see if we can make more general conclusions about how to use our new information to best protect areas of important biodiversity, by placing marine protected areas in places where species move least, for example,” he said.