The most recent issue of Science has two new reports on the so-called global warming hiatus and the carbon cycle in the Arctic, both of which are central to discussions of climate change and where temperature increases are being seen around the globe. I’d like to summarize the findings of each below, but I’ve also provided the links for each if you want more detailed information. On to the first one…
Who is holding on to all that heat? (Article Link)
Why have global mean temperatures not increased since the early 2000’s? This question is both bugging climate scientists and becoming the rallying cry for climate change denialists. Climate scientists know this does not mean global warming is not occurring, as has already been discussed previously on this blog, but they still want to know where the heat is going. Previous research has indicated that the Pacific may be sending loads of cold water to the surface, limiting warming of the atmosphere. Or possibly increased haze from pollution could limit the greenhouse effect.
Now, new modeling published in Science indicates that the Atlantic Ocean may be holding much of this heat not seen in the atmosphere. This is the first indication that the Atlantic is a major player in this reduced warming. The authors used a wealth of data stored since 1970 – millions of temperature measurements from buoys and ships around the world and at depths from 0 (surface) to 1500 m down. Since 2000, they find, the Atlantic Ocean has stored more heat 300 meters down or farther than all the other oceans combined! Here’s a nice little graph to show this:The top graph is the total heat content since 1900 across all oceans, and the bottom four graphs break up this content by ocean (time on the x-axis, heat content on the y-axis). You’ll see that, after 2000, the Atlantic heat content at several depths (different colored lines) increases dramatically compared to the Pacific or Indian Ocean (the Southern Ocean, partially connected to the Atlantic, also shows this increase.)
One mechanism behind this might be a natural ‘conveyor belt’ of warm salty water that moves north through the Atlantic Ocean, before cooling in the Arctic and sinking. This belt accelerated in the late 90’s before decelerating in the last decade, which might explain all this buried heat. More research will be necessary on that front.
I’t also interesting to note that a Nature Climate Change paper just came out this month that indicates a warmer Atlantic Ocean leads to warmer air rising over North America and fuels faster trade winds seen over the Pacific. So this Atlantic heating might be the major player in several climate phenomena we’ve been seeing recently. On to part deux!
Sunlight and carbon in the far north (Article Link)
You might not think that the Arctic is a major player in carbon release and resulting global warming. Yes, we know that glacial melting in or near the Arctic is a major issue for sea level rise, but that’s not all. New research indicates that photochemical processes are breaking down organic carbon and releasing carbon dioxide at a much faster rate than previously known in the Arctic.
This was unknown to me, but the Arctic is actually covered with many inland lakes, streams, ponds, etc. Globally, these types of inland water bodies only account for a few percent of continental coverage, and conventional wisdom said they were only important, regarding climate change, for transporting water to the oceans. But we now understand that these inland basins are filled with organic carbon, accumulated from nearby plant life in sediments, waiting to be broken down and have carbon dioxide released as a result. Bacteria break down this carbon material using ultraviolet or visible light as an energy source, and emit CO2 as their waste.
The authors of this paper actually went to the Arctic and took samples across many streams and rivers on the North Slope of Alaska. They then exposed these samples to light of different wavelength and observed how much CO2 is processed. Based on these CO2 rates, they scaled these estimates up for the entire Arctic, and determined that 70-95% of all reactions of organic carbon to CO2 are due to photochemical processes, likely due to bacteria.
This has major implications for both understanding and estimating how much carbon will be released in the northern regions of the globe. These are also much higher estimates than those previously found in non-Arctic regions. This may be due to the generally more shallow waters in the Arctic compared to elsewhere. This allows sunlight to reach all bacteria, whereas in deeper waters other forms of microbes degrade organic carbon without the help of sunlight.
Chen, X., & Tung, K. (2014). Varying planetary heat sink led to global-warming slowdown and acceleration Science, 345 (6199), 897-903 DOI: 10.1126/science.1254937
Cory, R., Ward, C., Crump, B., & Kling, G. (2014). Sunlight controls water column processing of carbon in arctic fresh waters Science, 345 (6199), 925-928 DOI: 10.1126/science.1253119