Tipping points are haunting uncertainties because they pertain to the unpredictable moment when the cumulative effects of environmental disturbance can trigger feedback loops of unstoppable change that can collapse entire ecosystems. They apply everywhere, from species loss and climate change to ocean acidification and food production. The best predictors are mostly intelligent estimates based on projected effects. Tipping points leaves scientists anxious because of the combination of uncertainty and extremely serious consequences.

We have, however, learned of the occasional tipping point from experience. The collapse of Newfoundland’s Grand Banks’ cod, for example, occurred when over fishing lowered populations below a critical threshold and a once stupendously rich resource simply vanished from commercial value. So we usually learn in retrospect what the critical conditions were. But by then the tipping point has been reached and the sorry end is a foregone conclusion.

Of course, we have warnings. But we never know how seriously to take them because we can’t be certain how to assess the risks. When does the concentration of open net-pen salmon farms in BC’s marine ecology create the conditions that undermine the viability of wild salmon? And when does the gradual loss of wild salmon trigger a fatal collapse in orca populations? We don’t know that either. That’s another experiment we are running.

On a larger scale, we know that plastic particles are accumulating in our marine ecosystems but we don’t know when their toxic effects will render species inedible or poison the entire food chain. We know we are disturbing the planet’s nitrogen cycle because ocean dead zones have increased from just a few to over 400 in the last two decades but we don’t know when this should cause alarm. We know that carbon dioxide emissions are increasing the production of oceanic carbonic acid but we don’t know when the acidifying process will trigger a chain reaction that collapses the entire structure of marine life.

This uncertainty is risk. So we need to decide what risks we are willing to take with tipping points. Climatologists estimate that we could court climate disaster if we raise biosphere temperatures above 2°C. We are already at 1.6°C, with global greenhouse gas emissions still climbing by record amounts and no serious constraints planned until 2020. Some climatologists argue that “thermal inertia”, the delayed response of climate to emissions, means we are already committed to a temperature increase of at least 4°C by 2050 and 5.5°C to 7°C by 2100.

If we don’t know exactly where most tipping points are, then how do we know when our behaviour is safe or foolish? We don’t know ‹ yet. But we already have evidence that large quantities of methane, a potent greenhouse gas held in storage by cold water and frozen tundra, are being released in alarming quantities as Arctic regions warm at a rate almost four times faster than elsewhere. And measurements indicate that methane is venting along the entire interface of the Arctic and North Atlantic Oceans. Both events worry scientists because this methane release suggests we may have already exceeded a crucial tipping point.

Now we are getting disturbing information about food production on a warming planet. Satellite images of maturing wheat can identify when green plants stop growing and turn to brown. Previous indications were that a 2°C temperature increase would reduce these crop yields by 30 percent. New data from surveying the Ganges Plain suggest the reduction is 45 to 50 percent (Island Tides, Feb. 9/12).

More exacting studies in the US confirm the tipping point for three crucial crops ‹ maize (corn), cotton and soybean ‹ is 29°C. Up to that temperature, plant growth increases. Beyond that, for each degree-day spent above 29°C, production falls by 0.6 percent. (A degree-day measures the intensity and duration of temperatures above 29°C.) US agriculture regions in 2009 spend 57 degree-days above that threshold; climate models predict 413 degree-days by 2100. This means an 83 percent reduction in maize crops. Even the most optimistic reduction of greenhouse gas emissions would incur a 30 to 46 percent loss by 2050 (New Scientist, Aug. 26/09).

We have used and settled our planet based on the opportunities provided by combinations of circumstances. We can’t shift traditional agricultural areas to cooler locations without the soil, water or terrain needed for the industrial production of food. We don’t have other oceans to fish. Forests are not easily moved. Warmer temperatures increase the activity of the hydrological cycle, causing weather extremes of more storms, more rain, more droughts, more wind. And even weather has tipping points, moments during gradual change when extremes suddenly occur ‹ this process, called “emergence”, has created a new and enigmatic branch of science.

Change means tipping points for us, too. When fish fall below a certain threshold, we suddenly stop being fishers. When trees die of disease, we suddenly stop being loggers. When crops no longer grow, we suddenly stop being farmers.

So far, our adaptive skills have allowed us to moderate the impact of these environmental changes. But global trends promise both a rate and extent of change that will be unimaginably swift, dramatic and pervasive. Multiple tipping points lurk almost everywhere in our uncertain future. We can be certain, however, that we won’t want to experience what they might be.