Growing Season is Longer

Getting Ready for Spring (4):

Spring is here, and in Ann Arbor it snowed again last Thursday. It will melt fast. It’s been a winter of snow, and fog, and flood, and thunderstorms. Almost like it has been “spring” all winter.

This series of blogs (see links below) started with the revision to the planting guidelines that was published last year. This is the little map that appears on the back of the seed packages, and it has shown a shift of the planting zones. Warmer zones are creeping northward. Also in those blogs was a discussion of birds migrating and plants blooming earlier in central Europe. Then, there was a discussion of spring time snow cover, first at a single station in Alaska, then with a perspective on larger geographic regions. Large portions of the northern hemisphere have seen a reduction of springtime snow cover. There has been some regional increase in springtime snow cover, especially in the Himalaya. As some pointed out in the comments this is, in fact, consistent with increased atmospheric moisture in a warmer atmosphere and increased participation. (Note, I said consistent with, not attributable to.) In the eastern half of North America there have been patchy increases of snow cover in the spring. (Assignment: What are the climate predictions for eastern Canada and northeastern United States in March, April, May?)

Let’s return to vegetation. Satellites are able to measure vegetation in several different ways. One of the most used parameters is the Normalized Difference Vegetation Index ( NDVI). This measurement takes advantage of the fact that leaves in active photosynthesis have strongly different reflective characteristics than non-leafy environments. Plants use solar radiation in a specific part of the spectrum for photosynthesis. ( Photosynthetically Active Radiation) Outside of this range plants scatter and reflect solar radiation. Hence there is a strong edge in the spectrum of radiation reflected by the Earth. This can be used to tell when leaves come out and start to grow.

In a 2001 paper in the Journal of Geophysical Research , Liming Zhou and colleagues looked at trends of NDVI between 1981 and 1999. These data showed strong greening in the northern hemisphere between 40 degrees and 70 degrees north. Springtime moved earlier and the start of autumn moved later.

Figure 1 is from the web site of Ranga Myneni at Boston University. He is one of the co-authors of Zhou et al. paper.



Figure 1: Increase in length of growing season in North America and Eurasia. (From the web site of Ranga Myneni at Boston University. After Zhou et al. 2001)

In this paper they document that spring, north of 40 degrees in North America occurs about 8 days earlier at the end of the record than at the beginning. (The uncertainty is + or - 4 days.) In Eurasia spring is 6 (+ or – 2) days earlier. In North America fall starts 4 (+ or – 3) days later and in Eurasia it starts 11 (+ or – 3) days later. The growing season defined on hemispheric scales has increased by 12 (+ or – 5) and 18 (+ or – 4) days in North America and Eurasia, respectively. As you can see in the figure, there is variability in the time series, but the trend, which is almost half a month, is clear.

Hence, we see that throughout the northern hemisphere the beginning of leaf growth, the loss of snow cover, and the behavior of birds is consistent with the temperature increase that has been directly measured. I assert that these changes at the seasonal transition represent a natural “averaging” of the variability associated with weather. This is a robust indicator of warming.

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Some interesting web resources:

Recent Changes in Vegetation (Arctic Report Card 2007)

Faster carbon dioxide emissions will overwhelm capacity of land and ocean to absorb carbon

A relevant lecture from my class

Blogs on spring getting earlier:

Getting Ready for Spring (1)
Getting Ready for Spring (2)
Getting Ready for Spring (3)
Jeff Masters blog on snowy winters