What has the change in temperature been over the last 150 years?
The black line is increased from normally around -three degrees Celsius in 1850 to
around +.5 degrees Celsius in 2005. This means there has been increase of just
under one degree Celsius, or about one and a half degrees Fahrenheit, in the last 400
years. Let's focus on the middle figure.
This goes much further back in time to before 800 AD, so over 1,200 years ago.
The black line is instrument data, and as you can see, it runs out.
But we have different sets of proxy data. The colored lines, that go back much
further. These proxies are less precise of an
instrumentator. They allow making assumptions, about how
the climate change natural processes. But they provide climate out of the data
that goes much further. So they're very important to our
understanding of those earlier periods of those climate.
From this data, we can see that different proxies vary, suggesting that there is
maybe error in this proxy data. Or that different places in the earth
experience different change in climates. Not all climate change is global.
Sometimes there are local climate changes from place to place.
Interestingly, the proxy data broadly agrees, with itself and with instrumental
data. The proxy data suggests that the past was
generally cooler than it is today. Between one and zero degrees Celsius, or
between one and two degrees Fahrenheit. Notice that there was a big change in both
the proxy data and the instrumental data around 1850 when the anomaly, instead of
being a negative number became positive. We see this big increase in the
temperature of around just under one degree Celsius.
The reason why global temperature has change so much since 1850, is because of
industrialization. The vast majority of climate scientists
believe that the addition of additional greenhouse gases to the Earth's atmosphere
has increased the Earth's temperature. Examples of greenhouse gases from
industrialization include methane and, crucially, carbon dioxide.
The Industrial Revolution became widespread in Europe in the nineteenth
century. But then spread to much of the rest of the
world. And in fact it's global today.
Industrialization requires fossil fuel energy sources and by using these sources,
by burning oil, gas, coal we've added carbon dioxide to the Earth's atmosphere.
As we can see in the figure, the amount of energy that we've used by burning coal has
increased steadily from the 1850s. But other energy sources have also become
very important. Particularly in the twentieth century,
natural gas and oil, and these have become ever-increasing contributors to the
Earth's natural greenhouse by adding carbon dioxide to the atmosphere.
Here we have a figure taken from an ice core.
This ice core trapped bubbles of air from the atmosphere over 400,000 years, and so
by measuring the concentrations of carbon dioxide in each of these bubbles, we can
see how the amount of carbon dioxide has changed.
In 1950, there was around 280 parts per million of carbon dioxide in the
atmosphere. Today, we're at over 380 parts per
million. And looking at this figure, you can see
that the level in 1950 was already quite high to begin with.
In fact, there is more carbon dioxide in the Earth's atmosphere today, then there
has ever been in the history of human life on Earth.
I should point out that there have been periods in the Earth's past before humans
had evolved that had higher levels of carbon dioxide than are currently
observed. Carbon dioxide in the Earth's atmosphere
has steadily increased in concentration over the last few decades.
This is instrumental data from the observatory in Hawaii.
The light blue line is the monthly values, or the dark blue one is the yearly
average. The yearly average continues to go up.
Notice that the monthly value also rise, can you guess why?
It turns out that natural processes are still involved in regulating the Earth's
atmosphere. The large northern hemisphere's deciduous
forests absorb carbon dioxide in the spring months, but then they emit it again
in the fall as leaves decay. This demonstrates that the natural system
also influences the composition of the earth's atmosphere but it also shows the
limitation of that natural system. The natural process of the earth can react
quickly enough to absorb all the additional carbon dioxide that human
activities are adding to the atmosphere. As we learned in previous lectures, adding
carbon dioxide, a natural greenhouse gas, to the Earth's atmosphere should change
the Earth's climate. We've seen this data for temperature in
this lecture. The Earth's temperature is warmer today
than it was 125 years ago. But, can you think of other examples of
how increasing carbon dioxide, which should increase the natural greenhouse
effect, which should increase the average global temperature, should influence the
Earth? Lets look at some examples.