July 2016 was the hottest July ever recorded. Same with June. And also May. April, though… April was also the hottest April. In fact, every month going back to October
2015 has been the hottest since 1880. Cue the Nelly. It’s hard to argue Earth isn’t heating
up. Because… unless you have a neck problem…
it IS. But what’s REALLY warming the Earth? The climate’s a complex system, influenced
by everything from our orbit to gases in our atmosphere to volcanoes. And when we say warmer… warmer compared
to WHAT? For most of history, temperature records looked
something like this… “Hot today. Hotter‘n’yesterday. Gonna be hot tomorrow.” But with the development of accurate thermometers
and standardized temperature scales in the 1700s, we could finally get some real data,
from ships crossing the ocean, weather stations around the world, hashtag colonialism. But weather isn’t climate, and a few temperature
records do NOT a complete global climate history make. It wasn’t until the mid-19th century that
we were collecting enough data in enough places to figure out an average temperature for the
whole world, which is why US climate graphs start here while the UK goes back to here. Should we believe what Ben Franklin or Thomas
Jefferson wrote in their weather journal? Can we really trust all those old records? These are good questions, but climate scientists
have looked at the data through many different lenses, and every time they do the math, they
get the same answer. Earth is warming up, fast. But none of this explains the cause. It could be human activity, but it could also
be so many other things. Earth’s orbit is pretty wobbly. Our elliptical path around the sun spins like
a hula hoop. Earth’s axis draws a circle every 21,000
years, and wobbles back and forth every 41,000, and all these affect Earth’s climate. But scientists understand these changes really
well, and when they use them to predict climate change? They don’t see any. The sun provides almost all of Earth’s heat,
and it also changes in cycles, dimming and brightening sorta like a light bulb. For most of the last thousand years, when
the sun turned up, temperatures rose on Earth, and when the sun dimmed, temperatures fell. But in the past few decades, the sun’s been
cooling slightly, yet Earth keeps getting hotter. Solar activity can’t explain today’s climate
change. We know carbon dioxide traps heat in the atmosphere,
and the last time Earth’s CO2 levels were this high, Homo sapiens didn’t exist. But maybe all this CO2 is Earth’s fault? When volcanoes erupt, they release tons of
the stuff, like little magma powered Earth burps. Geologists measured how much, and it turns
out humans release about a hundred times more CO2. Volcanic activity can’t explain climate
change. Some things actually cool the Earth. Cutting down more trees makes Earth’s surface
lighter, it reflects more light back into space. Clouds, pollutants, and aerosols in the atmosphere
do the same thing, they make our atmosphere reflective, so less radiation gets in. Yet even with these cooling effects getting
stronger, Earth’s getting warmer. But we know that greenhouse gases like CO2
and methane have skyrocketed, and when we calculate how temperatures should change based
based on those levels, we finally see a rise. Subtract the cooling from trees and clouds
and pollution, and it matches more than a century of data. This isn’t magic, it’s math. These climate graphs don’t show absolute
temperatures, because absolute temperatures, the number on a thermometer, can be misleading. The top of a mountain will always be colder
than the valley below, but if they are both a degree above normal, it gives us a hint
that the larger climate might be different. This is why we look at the “anomaly”…
how different today’s average temperature is from the average temperature somewhere
in the past, the degrees above “normal.” But what IS normal? The last month Earth’s temperature was below
the 20th century average was December… 1984. Many of you haven’t seen below-average temperatures
in your lifetime. July 2016 was the 379th consecutive month,
and the 40th July in a row, of above average temperatures. Yet you probably heard more news lately about
breaking Olympic records than climate records. Are we really just… used to this? Maybe… In 1995, biologist Daniel Pauly coined the
idea of “shifting baselines”, the idea that we evaluate change very differently depending
on what we’re comparing to. Today, about half a million bison live in
North America, a remarkable recovery from the late 19th century. But compared to the 20 to 30 million that
roamed the plains before 1600, our bison baseline suddenly paints a very different picture. We know how hot Earth has been over the past
century and a half. We have the data. Scientists understand that Earth’s climate
is a complex puzzle, whose pieces affect one another in many ways. When they put those pieces together, to recreate
that history, the picture is clear. Let’s take a long hard look at it. Stay curious.
