By Andrea Alban-Davies, Rye Garden Club Conservation Committee
“The thing is that small changes in average temperatures have huge effects on our climate.”
Just last week, I began planning an order for my spring planting. As is probably true of most gardeners across the country, I consulted the USDA Plant Hardiness Zone Map before making my decisions. The hardiness map is based on the average annual minimum winter temperature, is divided into10-degree Fahrenheit zones, and is the standard by which gardeners across the country can determine which plants are most likely to thrive in a given location. The latest version dates from 2012, when the whole map was shifted northward (generally by one 5-degree Fahrenheit half-zone) from the previous map dating from 1990, due to milder winters.
Practitioners in the field believe that the map was already out of date shortly after it was published. Because winter temperatures are increasing more rapidly than summer temperatures, one prominent academic made the case that the hardiness map would need to be shifted another half- to full zone for almost half of the country. What does this mean for our plants? Nir Krakauer, a professor at City College of New York who overhauled the USDA hardiness map back in 2012, contends: “Generally speaking, you can now grow varieties 100 miles further north than you could about 30 years ago.”
All of this information got me thinking… what, exactly is the significance of a degree? We hear a lot of talk about average global temperatures, average regional winter temperatures, peak temperatures, number of days above a certain temperature, and so on. Sometimes we hear about huge daily swings or extremely high or low temperatures, and other times we hear about the catastrophic effect that just a few degrees above pre-industrial levels can have on our planet.
The Earth has already warmed by roughly two degrees Fahrenheit since 1880, when we first began tracking such metrics. We also know that scientists generally agree that, in order to avoid disastrous climate change, we need to hold the increase in average global temperatures to some level below 3.6 degrees. Fahrenheit. But when we hear “2 degrees Fahrenheit”, it can be difficult to understand why such a small number can have such significant consequences; especially when our personal experience with degrees tells us to ignore such trivial numbers. After all, our local temperatures can swing 30 degrees or more in one day. Why do we need to be so worried about adopting sustainable habits and lifestyles in order to limit our planet’s warming by just a few degrees?
The thing is that small changes in average temperatures have huge effects on our climate. How do we know this? Because our planet can tell its own history; of particular relevance are ice cores, which allow us to understand accumulation, air temperatures, and air chemistry from another time, ocean cores, and the Earth’s own topography. Consider this: 20,000 years ago, near the peak of the last ice age, average global temperatures were, by most estimates, about 7 degrees colder than during pre-industrial times (or 9 degrees colder than they are today). Nine degrees – that’s it! And what did our country look like? Simply put, nothing like it does now. The Laurentide ice sheet, thousands of feet thick in places, covered much of Canada and the Upper Midwest and spread along the East Coast up to what is now New York City. About 100 miles off today’s coast of New York City, the Hudson River used to drop over the continental shelf into the seas below. All that is left today is the deep submarine Hudson Canyon, which tells us that the river once cut a path extending 450 miles across the shelf, eventually reaching a depth of 10,000 feet. In the West, you could walk from what is now San Francisco to the Farallon Islands. The list goes on and on.
So, when we think about the 2 degrees that we’ve already warmed our global climate since 1880 (an unprecedented change over such a short period of time), it now starts to sound like a lot — especially knowing that that figure includes the surface of the ocean, and that warming is greater over land, as the hardiness map makes clear. Perhaps even more concerning (and of particular importance to all coastal cities and towns – and humankind in general), warming is greater still in the Arctic and parts of Antarctica, where ice and ice sheets are cracking and melting into our oceans at an alarming rate.
Scientists tell us that, if emissions continue unchecked, global warming could exceed 8 degrees by the end of this century. Such a rise would transform the planet as we know it, crippling its ability to support a large human population. The panoply of disastrous consequences would likely include catastrophic sea-level rise and highly unstable food production. Our planet at 9 degrees cooler must have looked like the set of a sci-fi movie. An 8-plus-degree swing in the opposite direction would likely look similarly unrecognizable. The time to act is now.