If you question the numbers, ask the plants

The 2012 Plant Hardiness Zone Map unveiled by the US Department of Agriculture this week shows average annual extreme minimum temperatures based on data from 1976-2005.This version is modified to use the same colour code as 1990. See the end of the entry for original image and link to USDA interactive map. Credit: US Department of Agriculture/Friend of the Earth

You may have heard of dandelion clocks, but have you ever thought of looking at plants to check the temperature? It may not give you a precise reading, but changes in where plants can live in the US and when they grow in China have clearly demonstrated global warming this month. They reinforce the recently reported worldwide average surface temperature for 2011, providing a real world example of the climate change shown in scientists’ graphs.

Much of the US was one 5°F (2.8°C) half-zone colder in the 1990 Plant Hardiness Zone Map compared to the latest version. Credit: US Department of Agriculture

On Wednesday, the US Department of Agriculture (USDA) unveiled its latest map of planting zones, which has been redrawn to reflect warming seen since the last version was published in 1990. The Plant Hardiness Zone Map shows zones representing average annual extreme minimum temperatures. The old map was based on temperatures from 1974-1986, but updating it to include temperatures from 1976-2005 has shifted many zone boundaries. Though that’s in part due to new technology and better weather data, across much of the US the map is one 5°F (2.8°C) half-zone warmer.

And while the USDA says that the map is “not a good instrument” to assess climate change, David Wolfe, professor of plant and soil ecology in Cornell University’s Department of Horticulture says it’s being too cautious. “At a time when the ‘normal’ climate has become a moving target, this revision of the hardiness zone map gives us a clear picture of the ‘new normal,’ and will be an essential tool for gardeners, farmers, and natural resource managers as they begin to cope with rapid climate change,” he told Associated Press reporter Seth Borenstein.

Earlier springs, later autumns

The cycles in China during the 1960s (blue line) and a 1998-2007 (red line). Dashed lines indicate temperature thresholds for the 24 solar terms, which have risen during this period, making spring events happen earlier and autumn events later. Credit: Science China Press

Meanwhile, Chinese researchers described a similarly notable shift in phases in the seasonal cycle of crop growth in their country between 1960 and 2008. Historically the phases have been linked to 24 ancient solar terms timed by the position of the stars, like “Waking of Insects”, “White Dew”, and “Grain Rain”, and have been used to to plan agricultural activity. In a research paper in the January issue of the Chinese Science Bulletin Qian Cheng from the Chinese Academy of Sciences in Beijing and colleagues tracked the changes in the climate conditions each term is traditionally supposed to represent.

Qian’s team found that over this period the timings of springtime events have advanced by 6-15 days and Autumn events are now 5-6 days later. From 1998-2007 there were less than half as many days of conditions relating to the coldest terms as during the 1960s, and almost twice as many of the warmest phases. “The implication of the present study for agriculture is that under the climate change of earlier timings of the Waking of Insects, Pure Brightness, Grain Full, and Grain in Ear almost everywhere in China, agricultural activities need to be brought forward by several days,” the researchers write.

The US National Oceanic and Atmospheric Administration says that 2011 tied with 1997 as the 11th warmest year since records began in 1880, and was the warmest La Niña year. Credit: NOAA

Thanks to the cool phase of a global climate cycle, the warming underlying these happenings took a pause last year. This month NASA ranked 2011 as the 11^th warmest year on record, and the US National Oceanographic and Atmospheric Administration (NOAA) ranked it as the 9^th warmest. After 2010 had been the warmest year in these organisations’ records the world was visited by La Niña, the colder sister of, and part of the same climate pattern as, the warming El Niño. It was the warmest La Niña yet, however, half a degree Celsius higher than the average temperature for the 20^th century.

Despite this, I still heard British politician Nigel Lawson, who served as chancellor under Margaret Thatcher, arguing that the 2011 data show that there’s been no warming this century on the radio this week. This is a common argument used by those who argue that the climate isn’t warming, that works because they’re looking at short time periods, as the graph below shows.

It’s possible to see cooling trends in the record of global temperatures if you focus on short time periods, but the record as a whole reveals steady warming. Credit: Skeptical Science

It’s true that the temperature graphs used to show warming aren’t always easy to understand. But the temperature changes are having clear effects on the natural world, like where and when plants grow. These are effects that many of us experience directly – and provide a powerful demonstration of global warming beyond what tables and charts can achieve.

This is the 2012 Plant Hardiness Zone Map from the US Department of Agriculture with revised half zone colour coding. Credit: US Department of Agriculture

For the full interactive Plant Hardiness Zone map visit: http://planthardiness.ars.usda.gov/PHZMWeb/