State of the Climate: Earth’s Temperature

“Everybody complains about the weather, but nobody does anything about it.”

That now-famous saying is often attributed to Mark Twain, but in reality he only popularized it when, in a public lecture, he quoted its originator: his friend and sometime co-author Charles Dudley Warner.

These days there seems to be as much talk about climate as there is of weather, yet we have actually been doing something about it — unwittingly — for quite some time now. One cannot venture out of doors without at least some inkling of what the weather is doing, but few are aware of what state our climate is in, how it has changed, or even the proper statement of what climate is. It is to remedy that situation that we present, for your edification, this brief summary of the state of the climate regarding Earth’s temperature.

To begin our tale with the beginning of our tale, we offer this definition:

Climate is the mean and variation of weather over long periods of time (usually, at least 30 years).

Most of us already knew about the mean part, that climate is some sort of “average weather.” But just as important is the variation; if, for instance, two areas have the same mean temperature but one tends to fluctuate wildly between hot and cold while the other always keeps the thermometer at a steady reading, they have very different climates. To elaborate further, we add this:

Weather is the state of the atmosphere at a given time, especially regarding temperature, pressure, humidity, precipitation, and storminess.

Climate is therefore the mean and variation of all the quantities which characterize weather.

TEMPERATURE

Man-made climate change is often referred to as “global warming,” which underscores that not only is temperature an important part of climate, it’s one of the most prominent aspects of climate change. Although there’s much much more to weather than temperature, and much much more to climate change than warming, the most obvious answer to the question “what’s up?” with the climate is: temperature.

Mankind has recorded temperature in a few places almost since the invention of the thermometer in the 1600s, but only for about the last century and a half have we had enough thermometers over enough of the world to estimate global average temperature.

For the purpose of studying climate change, we’re most interested in how the average has changed and fortunately, the change can be measured more precisely than the average itself (just as we know with far more precision how sea level has changed, than how deep the ocean is). We also want to know how temperature changes apart from the march of the seasons; it’s surely hotter in summer than in winter, but is this summer hotter than summer typically? Therefore climate scientists focus on temperature anomaly: the difference between temperature at a given time, and what is usual (or at least, what was usual) for that time of year.

Here’s the global average temperature anomaly for each month from January 1880 through May 2016, according to data from NASA:

Here’s the same data from 1970 to now, a period during which Earth has warmed steadily:

The red line is the trend. That’s extremely important because it helps us understand what to expect.

It’s obvious from looking at the graph that temperature fluctuates — a lot, and in mainly unpredictable fashion. But there’s more going on than just the fluctuations, the “random noise” that makes up regular temperature variation; there’s also a trend. On the planet we call Earth, that trend lately has been steadily upward. The fluctuations have most decidedly not been steady! Nor can we expect them to be, ever. But we have every reason to believe — from theory, from computer simulations, and from observations — that the trend will continue. Upward.

The fluctuations represent weather change, change that is part of the climate itself (the “and variation” part), not climate change. Fluctuations are the constantly changing face of weather, and we expect those fluctuations to occur. All of which reinforces the old adage that “climate is what we expect, weather is what we actually get.”

But the trend indicates a change in the mean, of what we expect, which is indeed a change of the climate, not just weather. Hence we see the most obvious, and most talked-about, aspect of climate change: that the mean temperature of the Earth as a whole is going up.

The rise is a even clearer if we reduce the noise level by calculating the average over each year:

The temperature-so-far this year (circled in red) is far above what has been observed previously. But this year isn’t over yet; when all is said and done, its average is likely to be less than that extreme value but still the hottest year on record.

Part of its extremity is due to the phenomenon called the el Niño southern oscillation. During its el Niño phase heat migrates from oceans to atmosphere, making our weather a bit warmer overall. When it enters the opposite la Niña phase, as it may well do now that the el Niño is drawing to a close, heat migrates from atmosphere to oceans which makes our weather a bit cooler.

But it’s not the el Niño which made 2015 a record-hot year and is likely to make 2016 break that record. There have been el Niño episodes many times before but this one is hotter than all the previous, and what made it so is the upward climb of temperature which is evident as the rising trend. The trend is because of us.

The world has already warmed 1°C (1.8°F) above what it was before the industrial revolution, usually called “pre-industrial.” The prevailing view is that warming by 2°C above pre-industrial will bring dangerous climate change, although lately many are coming to believe that even 1.5°C above pre-industrial is dangerous. The 1°C we’ve already seen is trouble, in the form of more and more severe heat waves and wildfires, changing patterns of flood and drought, and coastal flooding from sea level rise — but 1.5°C or 2°C is the danger zone. Beyond 2°C is worse yet; many would warn that it brings disaster.

NASA isn’t the only organization that tracks global temperature. There’s also the National Oceanic and Atmospheric Administration (NOAA), the Hadley Centre/Climate Research Unit in the U.K., a modified form from the Univ. of York in the U.K., independent data from a team organized by researchers from Berkeley Univ. in California, and the Japan Meteorological Agency, just to name the best-known. It would be redundant to show you data from all these organizations, because they all tell the same story: lots of fluctuation, record hottest recently, and most important: the trend is going up.

UNEVEN WARMING

Undeniably the planet has warmed, but not by the same amount everywhere. This map (courtesy NASA) shows how much the 2010-2015 average temperature warmed (or cooled) relative to the 1951-1980 average, for various regions of the world:

Overall, the Arctic has warmed more than any other large region of the planet. This is just what was expected; both theory and computer simulations predicted it more than 30 years ago.

While the globe as a whole has shown an increase of about 1°C (1.8°F), Arctic temperature has risen by 3°C (5.4°F). Once again, the year-so-far 2016 is astoundingly hotter than any previous, but just as with global temperature we expect by year’s end that the Arctic average won’t be quite so extreme, but still a record-breaker.

Although only a few places have cooled relative to the 1951-1980 baseline period, one of those is an area of the Atlantic ocean south of Greenland.

Why would this region cool, in a warming world? It seems related to the fact that Greenland itself has gotten so hot that much of the ice sheet has been melting. The meltwater flows into the ocean, and it’s fresh rather than salty water, so it floats on top rather than sinking to the bottom and helping to drive ocean circulation. This has altered ocean circulation patterns, and some of the heat which ocean currents transported from equatorial regions to the north Atlantic isn’t going that way as it used to. Result: that part of the Atlantic ocean has cooled.

Another big contributor to uneven warming is the difference between land areas and oceans. This is easiest to show using data from NOAA:

While the whole Earth has warmed about 1°C (1.8°F) since pre-industrial times, the land areas alone have warmed around 1.8°C (3.3°F). This difference is even easier to understand; the oceans have huge thermal inertia, so it takes a tremendous amount of energy to heat them up. As a result they warm more sluggishly, and Earth’s land areas have warmed faster than the global average.

The lesson is that global warming is far more complicated than “every place warms at the same pace.” But overall, averaged over the entire Earth, we’ve definitely gotten hotter.

EFFECTS OF WARMING

What does climate warming do to the planet? One of the most obvious impacts is that much of Earth’s usually ice-covered regions are melting. This includes land-based ice (glaciers and ice sheets) as well as sea ice which covers parts of the Arctic and Antarctic near the poles. But again the real world is a bit more complicated.

The amount of sea ice in the Arctic has dwindled rapidly over the last several decades (monthly averages from the National Snow and Ice Data Center, NSIDC):

The decline is evident, but so too is the annual cycle, with more ice in winter and less in summer. Once more, anomaly to the rescue! We can simply compute the difference between each month’s value, and the long-term average for the given month, yielding this:

Just since 1979 (when satellites began measurements), we’ve lost about 1.77 million km^2 of Arctic sea ice — an area larger than the state of Alaska.

But the Antarctic tells a different story. The sea ice around Antarctica has actually increased, although not as much as it has decreased in the Arctic.

The two poles are dramatically different. The north pole is an ocean surrounded by the world’s greatest land masses, North America, Europe, and Asia; the ice at the very top of the world is sea ice, and has been disappearing rapidly. The south pole is a continent surrounded by ocean; the ice at the bottom of the world is in its land-based ice sheets, the biggest reservoir of frozen water in the world.

Melting of those southern ice sheets may be one of the reasons for the uptick in southern sea ice. The newly released water is fresh rather than salty, and when it reaches the ocean fresh water freezes more easily than salty. Besides that, climate change has altered patterns of air circulation around Antarctica, which may also have contributed to the changes in its sea ice extent.

It’s another illustration of the fact that things are not always so simple and obvious in a warming world. But when it comes to the world’s ice, the melting is outpacing the freezing by a huge margin. This is not just true of the Arctic sea ice, but of the great ice sheets in Greenland and Antarctica, and the vast majority of the world’s glaciers. All told, changes to the cryosphere — the frozen parts of Earth — are some of the most drastic and unambiguous impacts of planetary heating.

Yet another rising trend is of the height of the ocean itself: sea level. The melting of land-based ice puts more water in the oceans, and heating the oceans causes thermal expansion of seawater; both effects have caused the sea to rise. Here’s sea level since 1880, based on measurements by tide gauges around the world (data from leading sea level researchers John Church & Neil White):

In a now-familiar story, you can see fluctuations but there’s also a trend. Upward.

Note that the trend doesn’t follow a straight line. Sea level has risen sometimes faster, sometimes slower, but it’s faster now, and in fact is faster than it has been for at least 2500 years (perhaps a lot longer). Since 1993 we’ve been measuring the height of the sea surface with satellites, which tell this story:

Fluctuations. Trend. Upward.

The effect of sea level rise is already being felt. Coastal cities are now prone to flooding, not just because of storms and torrential rainfall, but simply because of a very high tide. Miami has already spent hundreds of millions of dollars trying to combat it. They’re also threatened by seawater intruding into groundwater supplies, making them unfit for drinking and agriculture. And it’s not just Miami, flooding has increased all up and down the U.S. east coast, from Maine to Florida and beyond.

What’s more, when storms and torrential rainfall do happen, sea level rise makes the flooding at coastal regions worse. A lot worse. The folks who live near the coast in New York, New Jersey, New Orleans, know what I mean.

Many other changes have been seen. Species are migrating to higher latitudes and elevations because areas that used to be the right temperature for them, are no longer. Plants are blooming earlier than before. Some places have become more prone to severe drought, while others are more prone to severe flooding. The list goes on and on; the changes shown include many of the most important, but really only represent the “tip of the iceberg.”

In case you’re wondering why all these changes are taking place, there are many reasons. But the most important, by far, is the fact that we’re adding greenhouse gases to the atmosphere. The one we’re having the biggest impact on is carbon dioxide (CO₂). Here’s the concentration of CO₂ in our atmosphere, in “parts per million by volume” (ppmv), measured at the atmospheric observatory at Mauna Loa in Hawaii:

There are fluctuations, which are dominated by a seasonal cycle. But again, there’s also a trend. This particular trend isn’t following a straight line; the rate at which CO₂ is increasing has been getting faster and faster.

The world is finally recognizing the need to reduce our emissions of CO₂ (and other greenhouse gases). But so far, its atmospheric concentration continues to rise at an ever-increasing pace. If we don’t stop burning fossil fuels at the rate we’re going, we’ll be destined to pass that 2°C threshold which is believed to be the “critical limit” for dangerous climate change. If we don’t stop soon, how much danger we’ll face from going far beyond the 2°C limit is something we really don’t want to find out.

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