2015 was the Hottest Year on Record

Latest year of data shows global temperature rise alarmingly

The data is in, and the news isn’t good: 2015 is officially the planet’s hottest year on record. And it not only beat the previous record (which was just last year), it smashed it, coming in at 1.6 degrees Fahrenheit warmer than the 20th century average. It’s also the biggest temperature rise we’ve ever seen, compared to the previous record. That’s bad news for wildlife struggling to adapt to rapid changes, people who’d prefer to avoid horrific tropical diseases, and anyone living near sea level.

If there’s an upside to today’s announcement, it may be that the huge spike in global temperatures in 2015 may finally put to rest one of the most pernicious myths of the climate denial industry: the contention that “the world hasn’t warmed since 1998.” The nation’s most prominent “conservative weekly journal” has said it. Bloggers and commenters internet-wide have said it. The Chairman of the Senate subcommittee that oversees NASA and the National Science Foundation said it just last month. Let’s take a look at why this particular myth persists.

First of all, note that in every attempt to make the case for a “pause” in global warming, one particular year keeps coming up as the reference point: 1998. This is not a coincidence. The world’s temperature made a big spike in 1997 and 1998. Take a look at the two graphs to the right, which plot the global temperature anomaly (in Fahrenheit) over recent years.

Notice that the trend line is steeper in the lower graph? They show the same data; the only difference is that the above graph starts in a very warm year (1998), so the trend line starts a tenth of a degree warmer and the rest of the line looks flatter—though it still points upward, and is certainly not as flat as the “it hasn’t warmed” argument would imply. It’s a little like telling yourself, “Whew! I haven’t gained much weight since that time I stepped on the scale right after Thanksgiving dinner!” If you start the graph just two years earlier, in 1996, the trend is steeper, because it starts with a cooler (or rather, less warm, year). But since the global temperature took another big leap up in 2015, you can see that the trend is now pronounced even if you use 1998 as your starting year (graph below). And if you look across the 135 years that we have good data, the upward trend is quite steep indeed.

In a nutshell, what we are seeing is that even though greenhouse gas concentrations are rising at a fairly regular rate, the temperature of the globe doesn’t respond in exactly the same way: the temperature graph looks more like a stair-step, making a big jump up in one year, and then leveling off or even declining slightly for a few years (see 1999, 2000, and 2001 in the graphs above), before jumping upward again. And in most of the years when the temperature increased to new record highs (1997-98, 2005, 2010, and now 2015), all have something in common: they are El Niño years. Let’s look at why this is important.

Most of the Earth’s surface – 71 percent – is covered in water. And water is very good at holding and transferring heat. That’s why a blacksmith cools a newly-finished piece in a bucket of water instead of just sitting it out to air cool. The world’s oceans are acting like a planet-sized blacksmith’s bucket right now, absorbing the vast majority of the extra heat we trap in the atmosphere year after. In fact, the amount of additional heat that’s been building up in the ocean dwarfs the change we’ve seen on land. During an El Niño event, the trade winds that usually blow east-to-west over the Pacific Ocean subside, allowing unusual levels of warmth to build up in waters off the west coast of South America. This large area of extra-warm water, located at the equator, is uniquely positioned to transfer heat back into the atmosphere, because moist tropical air rises high into the atmosphere, where it can spread over a large portion of the globe. Thus, global air temperatures spike in El Niño years, and generally fall back a bit over the next few years, as some of that heat is re-absorbed into the ocean. They never quite fall back to the previous low cycle, however—so the temperature rises in a “two steps forward, one step back” fashion, with the unfortunate side effect of giving the climate ostrich gang a new baseline from which to pretend the planet hasn’t warmed over the past few years.

The record temperatures of 2015 are also a stark reminder of just how momentous last month’s Paris Climate Agreement was, and how critically important it will be for the nations of the world to fullfill our commitment to address climate change in coming years. Wildlife and people depend on it.