Creeping Onset of Spring

Getting Ready for Spring (2):

If the planet is slowly warming, it is difficult to make that determination from the day-to-day and even the year-to-year weather observations. There is large variability of the temperature, plus there are sometimes problems with the quality of observations. Both of these are especially true if only a single place is considered, like an airport being surrounded by a city – or your backyard. Because of this strong variability, we rely on temperature measurements collected over many years and from many places. We also use temperature data from different types of instruments, and there are continuous efforts of quality control to determine and correct observational errors. This collection of observations for many years and from many places and from many sources is a way to accumulate information and to remove the variability that is random. Random variability, by definition, averages out to zero if enough observations are accumulated.

The previous blog was on spring coming earlier (Getting Ready for Spring (1)). If we are seeing a warming at the Earth’s surface, there are many ways that this warming is manifested. One of natural places to look is at the seasonal transitions. In the interior of the United States, the transition from winter to spring to summer is a time of large changes in the weather. It is cold, then warm, then cold again. Those in the Southeast, where peaches are grown, know the tension every year as the peaches bloom and a cold front is on the way with potential fruit-killing frost. It is time of severe weather when cold air from the north and west is brought next to warm and moist air in the south and east. This is weather carrying out its role in defining the Earth’s climate, carrying heat from the equator to the pole.

The seasonal transitions are also a place where we would expect to see a natural accumulation of the impact of a warming trend. If there is a trend, then over the course of several years one would expect to see the onset of spring, perhaps defined by the last killing frost, to move earlier in the year. The transition from fall to winter, the first killing frost, would move later in the year. This is a place where the trend is “accumulated;” it is a natural place that random variability is “averaged out” and the existence of the trend is exposed.

The figure from paper by Gian-Reto Walther and many co-authors, entitled, The Ecological Responses to Recent Climate Change, in the previous blog highlighted changes in Germany of birds arriving earlier and trees getting their leaves earlier. Also in that paper is a short summary of similar observations from around the world. The report from Working Group II of the IPCC does an extensive summarization of evidence of the warm season getting longer all around the world.

Edges, like the seasonal transition, are important in the climate and in measuring climate change. Aside from the seasonal transition another place to look for changes is in mountains. Since it gets colder at higher altitudes we can see whether or not frost-free zones are moving to higher altitudes in mountains. We can look at whether or not frost-free zones are moving farther north in the northern hemisphere. That is what the map from the Arbor Day Foundation in the previous blog showed. It is worth pointing out, explicitly, that in all of these cases we are looking at frost, ice, because the formation of ice impacts plants, and some animals, strongly. It kills the peach blossoms, the fruit. Remember the ice edges in the physical climate system, sea ice, where there are large changes in albedo associated with the freezing of water and thawing of ice.

Another place that ice on the edges is important is snow cover. This year has been very snowy in the northern hemisphere. That it is snowy does not suggest that it is colder. If it gets warmer, it does not mean that we no longer see freezing temperatures in places like Michigan. If it gets warmer there is more water in the atmosphere, and when there is precipitation there will be more precipitation, and if it is below freezing, then that precipitation will be ice and snow. From a climate point of view it is more important to look at snow cover in the late winter and early spring. Is the snow melting earlier?

This figure from NOAA’s Arctic Change Web Site shows the trend in snow melt at the station in Barrow, Alaska.



Figure 1: The date of snow melt from Barrow, Alaska adapted from Stone et al. (2002)



The Barrow station is shown by Stone et al. (2002) to be representative of the North Slope of Alaska. The green line is a fit through the first part of the data record and shows only a small slope in the melt date. The red line, through the later part of the record, shows a large slope, and the black line shows the slope for the whole, approximately, 60 year record. The black line shows a greater than 10 day shift in the melt date, which is between the slopes calculated for the green and red lines.

Also on the figure is the dashed line derived from a statistical model which investigates which geophysical variables explain the snow melt most fully. Wintertime snowfall, springtime temperature and cloudiness are the most important variables. These variables are strongly linked to circulation patterns, specifically, the Aleutian Low and the Beaufort Sea Anticyclone.

Returning to the discussion of the above paragraphs, the last 25 years of this record show enormous variability. There is a random aspect to this variability, and after averaging a strong trend is found. The attribution of this trend requires consideration of many other sources of information.

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Stone et al. (2002): Earlier spring snowmelt in northern Alaska as an indicator of climate change, J. Geophys. Res., 107, 10.1029/2000JD000286.