Changes in Net Primary Production (NPP, in green), a measure of the speed of the photosynthesis process is closely tied to the overall amount of CO2 in our atmosphere (red). For each rise in NPP, CO2 levels fell by a similar amount, and when NPP fell, CO2 rose. Credit: Maosheng Zhao and Steven Running
The blossoming hopes that plants will thrive as the world warms up have been pruned this week by measurements of how much CO2 is absorbed by species on land. Net primary productivity – which measures the speed of the photosynthesis process crucial to plants – fell by 1 percent from 2000-2009, researchers found this week. As photosynthesis turns solar energy, CO2 and water to sugar, oxygen and eventually plant tissue, it’s one way that the world keeps the greenhouse effect in check.
“We see this as a bit of a surprise, and potentially significant on a policy level,” explained Stephen Running from the University of Montana, Missoula. The surprise comes because a previous study had shown that between 1982 and 1999 net primary productivity (NPP) increased by 6 percent, which, Running explains, “suggested global warming might actually help plant growth around the world.” While 1 percent is only a small reverse in comparison to the earlier increase, it still means 550 million tonnes of carbon per year less are being taken into plants than at the beginning of the decade.
Together with his colleague Maosheng Zhao, Running analysed satellite images and measurements to calculate how the volume of vegetation had changed. They found that 93% of the change happened in the tropics, with 61% in tropical rainforests. Negative changes in NPP in the different regions of the world “were mainly caused by large-scale droughts” Zhao and Running wrote in the top journal Science yesterday. NPP in the northern hemisphere continued to increase, but was offset by a larger decrease in the southern hemisphere.
Over the past decade (2000 through 2009), periodic regional droughts and a general drying trend over the southern hemisphere reduced global terrestrial net primary production (NPP). The color image shows how rapidly NPP changed from 2000 through 2009. Negative values are depicted with red colours, meaning decreased NPP, or reduced plant growth, while green colors refer to increased NPP, or enhanced plant growth. The levels of red and green refer to the strength of the trends. Image courtesy of Maosheng Zhao and Steven Running, the University of Montana
“This is a pretty serious warning that warmer temperatures are not going to endlessly improve plant growth,” Running said. That warning is especially serious because it includes plants grown for food and fuel as well as forests and wilderness species. “Reduced NPP potentially threatens global food security and future biofuel production and weakens the terrestrial carbon sink,” Running and Zhao wrote. “Continuous global monitoring of NPP will be essential to determine whether the reduced NPP over the past 10 years is a decadal variation or a turning point to a declining terrestrial carbon sequestration under changing climate.”
Honing, not restraining, energy usage
The US public has its assumptions wrong on what steps can be taken to conserve energy, scientists have claimed this week. In a survey of 505 people across 23 states, Columbia Univeristy’s Shahzeen Attari and her colleagues found that most tended to focus on trying to cut how much energy they use. “When people think of themselves, they may tend to think of what they can do that is cheap and easy at the moment,” Attari said. “That is, keeping the same behaviour, but doing less of it. Switching to efficient technologies generally allows you to maintain your behaviour, and save a great deal more energy.”
Writing in the scientific journal Proceedings of the National Academy of Sciences on Monday, Attari and her co-workers pointed to previous studies recommending Americans switch to better household and vehicle technologies. These argue that “changing the selection and use of household and motor vehicle technologies, households could reduce their energy consumption by nearly 30 percent – without waiting for new technologies, making major economic sacrifices, or losing a sense of well-being”. This could make a significant impact on global greenhouse gas levels, as the United States produces 21 percent of the world’s CO2 emissions, with 98 percent of US emissions attributed to energy consumption.
The researchers asked the survey group an open-ended question, to “indicate the most effective thing they could do to conserve energy”. The group was then asked to estimate the energy used by nine devices and appliances and the energy saved by six household activities. On average the estimates the amount of energy used and potentially saved were around a third of the actual amounts.
The impact of changing vehicles that get more miles to the gallon was often under-estimated, which the team points out isn’t helped by the miles-per-gallon (mpg) measure itself. Bizarre as it may seem, a 10 mpg improvement in a less efficient car has more impact on fuel use than in a more efficient car, the scientists note. Other incorrect assumptions include that line drying saves more energy than changing a washing machine’s settings, and that using and recycling glass bottles saves more energy than recycling a can. Producing a bottle from recycled glass does save around a third of the energy it takes to produce one from new material. However recycling an aluminium can saves even more – 95% of the energy it takes to make a new one.
Attari and her colleagues therefore call for more effort to improve people’s awareness of how they can reduce their energy usage. “If people are uninformed, the substantial potential to reduce energy consumption may go unrealized,” they write. “It is therefore vital that public communications about climate change also address misconceptions about energy consumption and savings, so that people can make better decisions for their pocketbooks and the planet.”
| Behavior category | Curtailment (C) or efficiency (E) | Percentage of participants |
| Turn off lights | C | 19.6 |
| Conserve energy | 15 | |
| Drive less/bike/use public transportation | C | 12.9 |
| Change the setting on the thermostat | C | 6.3 |
| Change my lifestyle/not have children | C | 5.9 |
| Unplug appliances | C | 5.7 |
| Shut off appliances/use appliances less | C | 4.9 |
| Recycle | 4.2 | |
| Other (for behaviors only mentioned once) | 4 | |
| Education/think about my actions | 3.8 | |
| Use efficient light bulbs | E | 3.6 |
| Use efficient appliances | E | 3.2 |
| Use efficient cars/hybrids | E | 2.8 |
| Sleep more/relax more | 2.8 | |
| Buy green energy/solar energy/alternative energy | 2.6 | |
| Insulate my home | E | 2.1 |
| There is no way/I don’t know | 0.8 |
Categorized responses to an open-ended question about the single most effective thing that participants could do to conserve energy in their lives. People more commonly suggested changing their behaviour to reduce , or curtail, their energy use, than using more efficient technology.
