Jeong, J., et al., 2014. Intensified Arctic warming under greenhouse warming by vegetation-atmosphere-sea ice interaction. Environmental Research Letters, v 9. DOI: 10.1088/1748-9326/9/9/094007
We are told from an early age that the Earth’s plants are an extremely important part of our world’s ecosystem. In biology class, we learn that plants take in climate-changing CO2, and in return replenish our atmosphere with much-needed oxygen. We learn that deforestation is bad for the planet, and that if we continue to strip the Earth of our lush resources, the concentration of carbon dioxide in our atmosphere will continue to rise. But this is mainly a point of argument in places that have always been lush with plants. What about the places on the planet that normally don’t have greenery, such as the poles? As it turns out, the presence of vegetation at high latitudes can have negative effects on polar sea ice.
In particular, the Arctic has become a hot topic (excuse the pun) in the discussion of global warming, and with good reason; It is one of the most sensitive places on Earth in regards to its response to warming, as evidenced by large glaciers retreating up valleys and marine life adapting to a world of dwindling sea ice.
In a place such as the Arctic, there has traditionally been very little vegetation to speak of. However, with the recent increase in greenhouse gases and associated climate change over the last several decades, this region has actually become more habitable for vegetation. With warmer summers, the growing season has lengthened, leading to an increase in greenness of the Arctic tundra as new species of plants move ever farther North. Good news, right?
Not quite. A recent study led by Jee-Hoon Jeong from Chonnam National University’s Department of Oceanography in Korea, and other researchers from around the world, has discovered that increased vegetation activity at these northern latitudes can actually accelerate the warming of the ocean surface layer in the Arctic. The research team has developed a model that suggests that vegetation-atmosphere-sea ice interactions, influenced by already-high levels of CO2 in the atmosphere, can lead to an increase in transport of heat to the Arctic Ocean from new vegetation on high latitude landmasses.
Here’s how it works: warmer summers at high latitudes mean a longer time for vegetation to grow on the normally snow-covered or barren surface of Arctic landmasses. The main effect of this phenomenon is that the albedo, or reflectivity, of the ground surface is reduced. Albedo is a measurement of how much sunlight is reflected by a surface. Snow ranks as having high albedo, as bright white surfaces reflect a large amount of sunlight, leaving very little energy to be absorbed. Vegetated land on the other hand, ranks having lower albedo, due to the land surface being covered in darker colors, consequently reflecting less and absorbing more solar energy. In the perspective of a warming climate, new vegetation is bad news, as increasing vegetation at high latitudes leads to increased warmth and snow melt.
Other effects of increased vegetation at high latitudes are now being suggested. Jeong and others are using their model to see what happens when you take into account the changes in sea ice and the changes in the atmosphere. They found that enhanced vegetation activity affects their simulated warming model by increasing the surface temperature over landmasses by 3 to 5 degrees in the summer (Figure 1). This warming carries over from the summer months to the winter months.
This is surprising, because if the main effect that vegetation has is decreased albedo, and there is very little sunlight in the winter months, then something besides the decreased albedo must be causing the warming during the winter. There must be something else playing a role besides the decrease of albedo on the land.
After analyzing other factors in their model, such as radiation, heat transport, and clouds, they found that the vegetation activity during the warm months enhances heat flow to the lower atmosphere, creating a surplus of warm air. This heat is then carried north by the atmosphere. With the increased warmth over the Arctic Ocean comes more melting of sea ice during the summer months. Less sea ice means decreased albedo in the ocean, which means increased warming of its surface. In turn, the exposed, warmed ocean area releases some of its stored heat during the winter months. This positive feedback cycle can continue, feeding off of itself, as warmer winters lead into warmer summers and longer growing seasons, encouraging vegetation growth, and feeding back into the warming cycle.
Even with the interesting correlations between vegetation and decreasing albedo in the Arctic, the model does have some limitations; it did not include other important ecosystem components, such as biogeochemical feedback (the carbon cycle feedback), or the thawing of the permafrost (frozen ground), which could possibly compound the decreasing albedo problem. However, it is clear that the rise in atmospheric CO2 has led to an increase in vegetation at higher latitudes, which contributes to a positive feedback cycle of warming in the Arctic. Perhaps more plant life – in the wrong places – is not the climate-saver we thought it was after all.