As I have ridden through the center of Mexico over the past few weeks, I have stayed in many small villages. In these villages, almost every household has a plot of land that they use to grow corn or beans, and most families make their own tortillas. The mother of a family in Minita de Cedro remarked ‘no one buys tortillas here!’ as she showed me how she made tortillas. If there is extra corn, it is fed to the chickens (‘everyone has chickens here’ ‘do you own chickens?’), which are then also eaten or used for eggs.

   What does a changing climate mean for these farmers? The results from over a decade of crop modeling and estimates seem to agree: while the food production of the world as a whole may not change much, massive shifts in agriculture will result and the production of poorer, developing countries, which already have a warmer climate, will decline. In other words, climate change will have little or an even beneficial effect on the farms of the northern U.S. and Canada, but it will hurt these small farms in Mexico.

   Warmer temperatures in already warm regions generally decrease yields. A friend of mine at Stanford studied wheat production in northern Mexico, and he found that warmer years resulted in lower yields. And this appears to be the story of climate change and agriculture in the tropics — many places in the tropics are near their limit of heat tolerance, and a warmer climate will hurt crops. For instance, India recently experienced warmer than normal temperatures and also lower yields.

   On the flip side, plants use carbon dioxide to grow, and more carbon dioxide in the atmosphere may help some crops. This effect, however, is most likely to be strongest in northern latitudes, and not here, in Mexico. Also, most of the estimates of this CO2 effect (see graphs here ) are based on greenhouse experiments, and a number of field studies (such as the one I worked on at Jasper Ridge at Stanford ) suggest that the effects of higher CO2 on plants are less than expected. Also, the CO2 effect is going to be far less for corn than for other crops, and corn is the most important crop — both culturally and economically — here in Mexico.

   Global warming will likely result in big shifts in agriculture. For instance, the corn in the U.S. Midwest may be replaced by wheat, and Napa Valley may no longer be suitable for wine grapes. Switching from one crop type to another will take investment, and perhaps even genetic engineering to adapt crops to new climates. Rich northern countries will be able to make these investments, while developing nations will likely struggle.

   As I look at the agriculture on these hillsides in Mexico, where most of the knowledge is passed on from one generation to the next, I wonder how easy it will be to change the way these people grow their crops, or even the types of crops they grow. In Presa de Bravo, where I watched these two woman make tortillas (movie left — same movie as before), I asked how they had purchased the land to grow their corn. I was told it was owned by their ancestors, and passed down through the family.

   Finally, climate change may make weather more variable — meaning both more good years and more bad years, and fewer average years. If you are a subsistence farmer, you care just as much about consistent yields as you do about the average yield. In every town I stop in, I ask what they do if there is a bad year for corn. Francisco in La Mojada, who is going to sell that wheelbarrow full of corn stalks for $2 for animal feed (photo left), gave me the same answer that everyone else has given me. They have to buy food, and it is very hard. Many of the people move elsewhere when they can’t find work, and a huge percentage of the illegal immigrants in the U.S. are people from these small Mexican villages who were unable to make ends meet. (I asked in one small town how many people in the town were in the U.S., and I was told 10%.)

   Right now, there are 800 million people who do not get enough food every day (only a small number of these are in Mexico – most are in Africa or Asia). This a huge problem, and probably deserves its own continent-crossing-awareness-raising bike ride. Helping these people eat enough, though, is often a task of improving agricultural output in these poorer regions. Climate change will only make this more difficult.

   The butterflies I visited last week congregate in the millions at the tops of mountains in the center of Mexico. They choose these places because they have the perfect conditions for surviving the winter — conditions that are now threatened.

   The butterflies that are wintering here are mostly hibernating. While the skies were full of butterflies, there were even more (believe it or not) hanging motionless on the trees. What they need is a cool dry climate to survive these winter months, before returning to northeastern North America in the spring. (Read more about their life cycle.)

   According to a recent study (read a less technical summary here), it is likely that in this region of Mexico, there will be more winter rain. Winter rain is currently extremely uncommon (like rain in the summer in California). It is cold at 11,000 feet, and moisture with freezing conditions kills the butterflies. In 2002, a January storm killed over 70% of the butterflies, and conditions bad enough to destroy all butterfly habitats may be prevalent by 2050.

   This study relies on predictions of future rainfall, which are not as reliable as predictions of future temperatures — the rain predictions may be wrong. If the butterflies have to move to a new place to find conditions where they can survive the winter, however, they may be in big trouble, as people are likely to be already living there.

   Indeed, the butterflies are threatened by deforestation as well, most of it illegal, as this region is, in theory, protected. On the ride up the mountains, I saw a number of electric saws turning trees into furniture.

   It is more complicated. The people living near the butterflies are poor. The trail entrance to the butterfly sanctuary was full of shops pedaling trinkets and young children begging for money. It was actually the least safe place I have felt yet on this trip, and I asked to camp inside the park instead of at the entrance where most people camp. Camped less than a kilometer from the butterflies, I heard a subwoofer down the hill beating out a baseline, suggesting that the area was not well protected. The next day, I was surprised how poorly maintained the trail to the butterflies was, as thousands of tourists trampled random trails down a steep mountain side.

   It is tough to see poverty next to natural wonders that need to be protected. I am sure that there is a win-win situation – one in which the people are paid more to protect the forests than they are paid to cut them down. It is hard to believe that the value of logging is higher than the value of seeing these butterflies. However, after witnessing the disorganization of the park, I worry about our ability to protect the place, and I also wonder what the future of the begging children is.

   And there is the challenge. We must conserve these forests, help these poor people, and reduce our carbon dioxide emissions if we want forests of monarch butterflies for our children (as well as better lives for the locals). The ecosystems of the world face these multiple challenges, and this is just one of the places like this I will be visiting.

   I’m in Mexico City now, enjoying biking among 20 million some people. You can see a sneak preview of shots from the city here.