At the 1996 international climate talks in Kyoto, a specific goal for minimizing the harmful effects of climate change was selected—keep warming within 2°C of preindustrial levels. Given that the globe has already warmed almost 1°C, meeting that goal was clearly going to take some serious work. The lack of real progress in reducing greenhouse emissions since then doesn’t exactly inspire optimism about meeting that goal.

But does the 2°C goal truly represent a minimal impact? A study published this week in Nature shows that policy makers would do well to consider specific goals for more than just global average surface temperature. If they do, they might find that even greater emissions reductions are warranted.

The study examined five targets beyond global temperature: sea level rise caused by thermal expansion (not melting ice), ocean acidification in the sensitive Southern Ocean, a measure of acidification elsewhere that would harm coral reefs, reduction of cropland productivity, and carbon lost from cropland soils. The numbers that were used for each of these targets are actually unimportant. The point was to discover whether having multiple targets could change the emissions reductions necessary to hit them.

To do this, the researchers needed to describe outcomes probabilistically—the odds that a goal is met by each of 55 emissions scenarios, taking into account all the relevant uncertainties. That means they needed many thousands of climate model simulations. The most complex climate models run far too slowly to accomplish all that, so they used a simpler type of model designed for this kind of work.

The results of all those simulations showed that some of their goals were much easier to achieve than others. The chances of keeping the thermal expansion of seawater below 40 centimeters in 2100 were pretty good (though it would continue to creep upward in the next century). It was also likely that the productivity of most of the world’s cropland would decline by less than 10 percent, as the benefits of higher CO 2 for plants helped offset some of the warming impact.

Other targets were less forgiving—namely ocean acidification. For scenarios where atmospheric CO 2 exceeded 550 parts per million (it’s currently just below 400), at least 90 percent of the world’s oceans became unfavorable for coral reefs. Other organisms in the Southern Ocean might be slightly better off. At around 600 parts per million CO 2 , calcium carbonate in roughly 20 percent of the Southern Ocean would be undersaturated, making life difficult for the plankton at the base of the food chain.

While ocean acidification depends purely on the concentration of CO 2 in the atmosphere, global average temperature depends on other greenhouse gases, as well. There are scenarios, then, where temperature could be kept below the target while ocean acidification goals would not be met. Likewise, a measure of ocean acidification might successfully be kept down while temperature exceeded the chosen limit.

When you decide that you want to meet all the goals at once, things get a little more difficult. It’s not just a matter of staying below the goal that is hardest to achieve. To have a high probability of meeting all goals, allowable emissions are at least a little lower than for meeting any individual goal on its own.

This provides a few take-home messages for policy makers. First, CO 2 will have to be considered separately from other greenhouse gases to account for ocean acidification, as well as warming. And, more generally, many types of targets should be examined together. Only a relatively complete accounting of the effects we're aiming to avoid will make clear what sorts of cuts we'll need to make. That includes economic goals and priorities.

Once a complete blueprint is drawn up, it should be possible to build an effectively coordinated plan to achieve the desired outcome. The approach described in this study is geared toward getting those blueprints right.

Nature, 2013. DOI: 10.1038/nature12269 (About DOIs).