Welcome to this next lesson about more recent changes in climate. We've had a look at the past million years of Earth's climate, and now we're going to check out some data from the more recent past, the last couple of hundred years. We'll look at some of the primary metrics of climate change-- like temperature, and sea level, ice cover, and greenhouse gas concentrations-- and how these things have measurably changed over time.

We're going to look at data, since it's good to get practice examining data in any realm of science where you want to learn more. So we'll check out some recent data, then we'll bring the past million years or so back into the picture again for a comparison. It's only recently that we've been collecting direct observations of climate metrics like temperature. The surface thermometer record goes back until about 1880. The satellite record goes back only until the 1970s.

And prior to those times we rely on data from other sources, like samples from ice sheets at different times in the past or the chemistry of deep ocean sediments. But here's the data for global average surface temperatures since 1880. You can see that there's variability from year to year-- that's normal and expected. For example, 1998 was an unusually warm average year because there was a large El Nino event. And during El Ninos, large parts of the tropical Pacific Ocean are warmer than usual, so the global average temperature is also warmer than usual.

But putting aside the year-to-year wiggles, the trend for the past century has been toward warmer temperatures. Temperatures have increased about 0.6 degrees Celsius in the last 50 years, about 1 degree Celsius in the past century. Recall that during the last ice age temperatures were five to six degrees cooler. That last image was global average temperatures-- one point per year for the entire globe. Here's another way of looking at temperature changes over time.

This global map shows the rate of surface temperature change in degrees Celsius per decade for data from 1979 to 2005. With this map we can look in greater detail at different regions. For example, where I live-- in Vancouver, Canada-- during this period my area got warmer at a rate of about a tenth of a degree Celsius per decade. You can find where you live and see the approximate rate there. If you're not very familiar with looking at maps like this, one thing to look for is where are the extremes-- where's the darkest red--and what numbers coincide with the dark red areas? Where's the darkest blue and what does that mean? Then beyond looking at particular regions, what are the larger patterns evident in this map?

Notice how there are more darker red areas over land. Just about everywhere, over land, has been warming during this period. Some of the strongest warming is at high northern latitudes. In the ocean, less area has been warming and in some places the sea surface has cooled. In general, with more land, the northern hemisphere has warmed more than the southern hemisphere. Let's break this down in time a bit more. First, let's look at a global map of average surface temperatures in the 1970s compared to a base period, which is the average surface temperatures from 1951 to 1980.

This base period is a logical and fairly common one to choose, because during that time the measured global surface temperatures didn't trend upward or downward very much. OK. So first, the 1970s. You'll notice that the 1970s are actually part of the base period, so one wouldn't expect them to be very different from the base period. And they weren't. The map shows some places that were little warmer in the 1970sthose are the yellow areas-- and some places that were a little coolerthose are the blues and the greens. OK.

Moving onward, here are the 1980s. This decade is a little warmer than the base period, almost everywhere. The 1990s are warmer still, and the 2000s warmer again. The 2000s are on average about 0.5 to 0.6 degrees warmer than the base period. Notice again that the northern high latitudes have warmed more and faster than other parts of the world. One of the reasons I find this display of the temperature data compelling is that I was born in the late 1960s just before the beginning of the data shown in these images. So these maps essentially represent the changes in global temperature over my lifetime.

Maybe you've seen the equivalent of two of these panels, or three, or maybe you've see a few more decades than are shown here. And I might-- and you might-- get the opportunity to see a few more. What are those feature maps going to look like? So surface temperatures on Earth have been warming, but they haven't actually been warming as fast as they might if we didn't have vast oceans on the planet. In a previous lesson we mentioned the heat capacity of water in the context of the amount of energy it takes to heat the water in a tea kettle.

Compared to other substances, water has a fairly high heat capacity, which means it takes quite a lot of energy to heat it up. In recent decades, the inflow of energy to Earth has been greater than the outflow. Just as in any system, if we have an imbalance of flows the stock-- in this case the stock of energy-- will change. On Earth where has the extra energy been going? Well, a lot of it has been going into the oceans. Here's one estimate of that.

On the horizontal axis we're looking at time starting in the 1960s. And on the vertical axis we're looking at the change in total heat content since 1961. The units on that vertical axis are 10 to the 21st joules. So add 21 zeros onto the ends of those numbers there on the axis. Over this time period, a small portion of the extra energy has gone toward heating land surfaces, the atmosphere, and ice. That's what's indicated by the lower shaded part.

But a much larger amount has gone toward heating the ocean. In addition to the helpful feature that water has a high heat capacity, the water in the oceans also mixes. The water on the surface mixes with the water underneath, taking energy downward away from the surface. Thus, it's not just the surface that's heating, it's also water deeper down, which means it takes more energy and longer to raise the temperature of the oceans.

Measurements show that the warming of the oceans extends down to at least 2,000 meters water depth. It takes a lot of energy to heat up a two kilometer deep bathtub. These data show that the climate system has been accumulating energy, as energy inflow has exceeded energy outflow for a while now. So the oceans are absorbing energy and heating up. But there's another additional response to this heating that's really important. When water heats up, it expands in volume.

So imagine the entire ocean heats up and thus the entire ocean expands in volume. Well where's the extra volume go? Since the bottom of the ocean isn't changing very fast, the only place for that extra volume to go is upward, raising the water level. This expansion of ocean water is responsible for about half the global sea level rise. And melting ice on land accounts for the other half. Now the cool thing about making predictions or forecasting future changes is that then time passes and we can see how good those forecasts actually turned out to be.

In this figure, the grey area represents the range of possibilities that seemed probable to the Intergovernmental Panel on Climate Change with the information they had when they did their assessment report back in 1990. And then time passed and people continue to monitor sea level. And it turned out that the 1990 estimate from the IPCC was quite conservative. The real future sea level has followed the upper edge of the projections made back in 1990.

Sea level is a metric that really matters on a practical level for many people, since a whole lot of us live very close to sea level. Some of us already live below sea level and we have barriers to protect our communities. That's certainly one of the adaptation options we might choose. Another, of course, is to relocate to higher ground. But that can present a major challenge in places with lots of human infrastructure close to sea level. Let's use these data to make an estimate of how fast global sea level has risen since 1990.How many centimeters of sea level rise happened between about 1990 and 2010? You can extrapolate a little bit to 2010.

In those two decades, sea level went up by a little more than 6 centimeters. From those data, the rate of rise turns out to have been about 3.1 millimeters per year. This is, of course, a global average. You might actually live in an area where locally sea level is falling because the land surface you live on is rising. Or you might live in an area where sea level is rising faster than the global average if the land you live on is subsiding at the same time.

It's worth finding out what's happening where you live. As a comparison, during the transition from the last ice age to the present warm period, the rate of sea level rise was sometimes much higher than our present 3.1 millimeters per year, up to 10 millimeters per year or higher. These higher rates of change in the past were associated with times when the major ice sheets had episodes of fast collapse, sending lots of ice and water into the oceans. Here's an image of one of our major ice sheets today. All of the major ice sheets, both on Greenland and on Antarctica, have been losing ice mass in recent decades. They gain some through snowfall, but they lose more through melting, which decreases the total stock of ice.

The net loss of ice on land contributes, as mentioned previously, to sea level rise. Sometimes large portions of the major ice sheets can undergo melting simultaneously, as happened on Greenland in the summer of 2012 when 95% of the surface of the ice sheet was melting at once. That's the grey area on the image on the right. Melting the surface produces puddles of water and those puddles are darker than the white ice, and therefore absorb more incoming solar radiation, helping the melting along. Just as a reference to think about, if all the ice on Greenland were to melt, we'd see a rise of about 6 meters of sea level.

And if all of the ice in Antarctica were to melt, we'd see another 17 meters. Nobody's forecasting that kind of catastrophic change happening quickly, but it's with imagining what your area would look like with even an additional 1 to 2 meters of higher sea level. Another type of ice, floating sea ice, doesn't influence sea level rise, but it does play a role in the flows of energy in Earth's climate system.

Sea ice is pretty thin, and it floats on the ocean. Every year a lot of it melts in summer and grows back in the winter and some sea ice sticks around from year to year, though that's becoming less common than it used to be. In the summertime when sea ice melts, it exposes the darker ocean water underneath, so during the melting season as the sea ice coverage declines the ice-albedo feedback helps it melt further and faster because the darker ocean, more of which is now exposed, absorbs more incoming solar radiation. Annually, the sea ice in the Arctic reaches its minimum extent in September, which is at the end of the melting season, and reaches its maximum in March, at the end of the dark winter.

In 2007, many people were taken by surprise when the sea ice extent in the Arctic Ocean decreased by more than had been anticipated to a record low. Compare the area covered with ice in this image for September of 2007 to the pink line, which is the median ice edge for September for all the available years of satellite data. 2007 had a much smaller ice extent than the median.Let's look at sea ice extent on a plot over time. Here we have data from 1980 through 2012. On the vertical axis, we have Arctic sea ice extent in September, which is the month at which sea ice is at its minimum. You can see the strong decline that happened in 2007, taking September sea ice extent down to about 4.3 million square kilometers.

Then just five years later in 2012 the sea ice extent record was broken again, and the September extent went down further to just 3.6 million square kilometers. Given the observed rates of change, there's a decent chance the Arctic will be virtually ice free in summer within my lifetime. Moving to the atmosphere now.

Here's the atmospheric carbon dioxide record since C. D. Keeling began collecting data at Mauna Loa in Hawaii in the late 1950s. Back then the value for atmospheric carbon dioxide was about 315 parts per million. Since then the value has been increasing, approaching 400 parts per million at the time this lesson was made. Plus the rate of change in the past couple of decades i s faster than the rate of change was in the first couple of decades.

The reason atmospheric carbon dioxide is rising is because the inflow of CO2 to the atmosphere from fossil fuel burning, land use change, and cement making, exceeds the outflow of CO2 from the atmosphere into the ocean and into land-based biomass and soils. In addition to the overall upward trend over time, notice the little wiggles. These are seasonal cycles related to plant growth and decay, which we'll look at in more detail later. And here's some interesting data.

Since the early 1980s, which is when people started making these measurements directly from air samples, atmospheric methane concentrations rose, then they plateaued, and then they started to rise again. Here's a stock and flow question for you. During which time period were inflow and outflow of methane to and from the atmosphere closest to equal? Was it about 1984 to 1990? Or 1999 to 2005? Or 2008 to 2013?Or can we just not tell from looking at these data?

During a time period when inflow and outflow are approximately equal, that means the stock isn't going to change much. So the middle time period, about 1990 to 2005, is the best of the given choices for inflow to equal outflow. These measurements then raise some interesting questions.What happened? What you'd have to do is figure out all the major inflows and outflows of methane and see how they changed over time. You'd have to look at more data or maybe even make more measurements. The key point I want to make here is that, in order to understand what's happened, we have to reconcile changes in stock, like these, with measurements of changes in inflow and outflow processes.Looking a bit farther back in time, it's evident that the recent rises in both these greenhouse gases, that is carbon dioxide and methane, all happened quite fast in quite short periods of time compared to the trends over the last 10,000 years prior to the Industrial Revolution. The Industrial Revolution is the sharp rise on the right of both of these graphs.

And to step back for a longer term view again, as we saw in a previous lesson, atmospheric carbon dioxide today far exceeds any concentrations from at least the past 800,000 years. So those are some data, measurements of stuff related to climate change in the recent past. These data are incredibly important for monitoring what happens over time. If Keeling hadn't started collecting and measuring atmospheric carbon dioxide in the late 1950s, or if that monitoring program had been dropped later, we wouldn't have this type of evidence to examine.

Satellite data starting in the 1970s greatly expanded our abilities to look at the planet and measure global changes. These kinds of measurements are crucial to continue and to expand if we want to continue learning more about how Earth's climate system works. And last, there's good evidence to link many of these recent changes to human activities, evidence we'll examine in more detail in a later lesson.