Agricultural Impacts

Plants need CO₂ to grow, and higher CO₂ levels will give faster plant growth. But, plants need many other things, too; in experiments with extra CO₂ added to natural ecosystems, an initial growth spurt lasts a few years before settling down to only slightly faster growth than before the CO₂ addition, because the plants need more of those other things to sustain fast growth. If CO₂is added to farm plants that also are supplied those other things, faster growth can continue, but the gain is still not huge.

Working against this fertilization effect of CO₂, the projected increase in floods and droughts would make farming more difficult. Farmers have learned to handle the bugs and weeds that now annoy them, but changing climate allows new ones to invade.

Perhaps the biggest concern is heat stress on crops. At present, anomalously hot weather reduces crop growth in many agricultural regions even if the plants have enough water, fertilizer and protection from bugs and weeds. For much of the world, continuing our present path until late in this century is projected to give average summer conditions hotter than the hottest summer up to 2006 (the last data available for an influential study). Record highs are rising with average temperatures, and expected to continue doing so. Thus, unless crop breeders become highly successful at developing heat-resistant varieties, heat stress may become quite damaging if we cause large warming.

Video: Net Photosynthesis (1:19)

Click for a video transcript of "Net Photosynthesis".

PRESENTER: This is from the site of the United States Department of Agriculture. Maize, which is what's shown here, is corn. And we eat a lot of corn and a lot of animals eat a lot of corn. Net photosynthesis is good. This is turning sun's energy into something we can eat. And so, up here, is eat and down here is starve. We don't eat if nothing is growing. And this is temperature, going from fairly warm to really hot up here, and in degrees Fahrenheit, there's 100 right there.

And what you'll notice is for this particular one, if you look at the rate of photosynthesis, what grows as it's affected by leaf temperature, when the leaf gets hot, growth slows a lot. This is something that's worrisome. In the modern world, a lot of places where we grow corn and other crops, on the hottest day of summer they don't grow very well because it's actually too hot for them. And we face some possibility that by late in the century, the hottest summer that we've ever seen until recently will be considered cool. And given this trend, that is something that a lot of people worry about.

Net Photosynthesis Data from laboratories (including the single example shown here) and the field show that above some level, plant growth is greatly reduced by warmer temperatures, and this effect is understood based on how the chemistry of plants behaves. In this figure for corn (maize) from the US Department of Agriculture, “Net photosynthesis” is plant growth, and clearly decreases as temperature increases.

Source: Figure modified from US Department of Agriculture, Learn About Our Plant Physiology Research.

Note also that the tropics are the big belt around the middle of the Earth, the polar regions are the small caps on the ends, and mountain ranges taper to points at the top, so simply moving poleward or up the mountains to follow cooler conditions involves losing ground. In addition, we now grow mid-latitude crops in soil that was transported by glaciers from higher latitudes or altitudes, so moving poleward in at least some places leaves most of the soil behind. Greenlanders are doing a little farming in special places such as on raised beaches from the ice age, but much of Greenland is too rocky for good farming, as shown below. So if Greenland's ice melts, raising sea level about 7.3 m (24 feet) averaged around the globe, and flooding valuable coastal property, the land revealed beneath the former ice sheet is not likely to be a wonderfully fertile replacement.

Video: South Greenland (1:15)

Click for a video transcript of "South Greenland".

PRESENTER: This is a small farm in Greenland. The ocean is out here where the big O is. And in here there are some grass-- there's some alfalfa that's being grown to feed a few sheep. When the glacier was here-- you know 20,000 years ago-- the weight of the ice had pushed the land down. And when the ice melted, the ocean actually had succeeded in flooding this area for a little while before the land came back up. And it put in a little bit of sediment, which the crops now grow on.

However, all of this stuff up here is just one rock sticking out, because the glacier cleaned off all the soil there. And you can't grow anything there and you'll see why there is there's no crops up there, because it's just hard rock. A whole lot of Greenland the looks that way.

So you know here's a person for scale, you are not going to grow crops on this. The glacier took the soil away. And so in case you hear anyone say, oh, when it gets too hot in the tropics, we'll just move towards the poles and we'll grow crops where the ice used to be in Greenland-- no.

Glacially eroded bedrock, east Greenland. Greenland farmers are raising a few crops and pastures, as shown here in south Greenland. The weight of the ice-age ice that covered the entire island pushed the land down. As the ice age ended, the ice melted faster than the land bobbed up. The extra water flooded the coast as the ice melted away, depositing ocean sediments especially along beaches. Then the land rose, and those sediments are exposed just above the sea. Farming is especially concentrated in those areas, as shown here. Notice, though, that almost everything else you see is hard rock that the glaciers polished clean, with little or no soil. The second picture in the video more clearly shows bedrock scratched and polished by the glacier, and not at all good for farming.

Source: Richard B. Alley

Overall, the effects of the rising CO₂ and the changing climate are expected to be mildly positive for farms for the near term, switching to negative and becoming increasingly worse beyond a few decades. One study found losses for US corn and soybeans of 30% to 82% by late in this century, depending on the scenario used and other factors.