I'll start by talking about how we know we know. 
How do we even measure past climate change?
 
One of the most well-known examples 
is ice cores. Has anybody here heard
anything about ice cores before? Yeah. A
lot of people. Ice cores are very
popular in Boulder because people 
here like snow and ice
I really appreciate that. So the way
those work, for those of you who don't
know is that snow falls and it accumulates and layers through time. And then you
can walk out onto the ice sheets in
Greenland or Antarctica, or you can go
to glaciers on the tops of mountains. 
You can basically stick a tube-- well, not
a tube, something like a tube-- into the
ice and pull out a core. But what do you
need in order to look at ice cores? You need ice, right? So as a tropical climatologist,
that doesn't really help me very much.
 There are tropical glaciers,
there are great ice core records from
Mount Kilimanjaro, for example, but we
need more than that. The other thing, too,
 is that if we want to look at very long timescales,
ice, while it is wonderful, it melts.
So I look instead at mud. Or we
call it -- mud, yes, you heard me right,
M-U-D mud. We call it sediment, that is its more technical term. So I look at
sediment that collects at the bottom 
of the ocean, or it collects at the
bottom of lakes. And what that
essentially is, is a huge repository
of everything that washed into that 
lake and settled out of the bottom. It's also
a repository for organisms that lived
within the lake, lived and died
decayed, settled out into the bottom. And
so just like an ice core, we can take
a boat out onto a lake, we can punch a hole 
in the mud, and pull out very long cores.
And what we end up getting is old mud at the
bottom, younger mud at the top. We can radiometrically
date that mud. And we can do all kinds of
chemical analyses. We can look at the
chemistry of the mud, we can look at
clay minerals have been washed in,
we can look at minerals that have been blown in,
we can look at fossilized pollen grains
produced by trees and grasses around the
landscape. We can look at particular
organic molecules, which is what I really
focus on, "biomarkers," which are indicative
of certain types of plants living on the
landscape, and tell you all kinds of information.
Lake Malawi has been around for probably more than
a million years. So it has been sitting there on the landscape, and its lake level
has been fluctuating a lot. And the sediments record that. Those layers are really nice, and preserved very
intact. And that's because Lake Malawi today, because it is 700 meters deep, has a layer at the
bottom that has very little oxygen in it. 
So there's not a lot of critters in there
to burrow through the mud and
mess up the nice laminations that we have.
A very different story is this other
picture here, this one on your right. This is
indicative of a low lake level. Most of
that mud is composed of calcium
carbonate, and that means that the lake
level had dropped so much that all the
calcium ions and the bicarbonate ions
floating around were able to fuse
together, form calcium carbonate,
and settle out and get deposited 
into the sediments. We also see really
no nice layers in that particular
sediment and that's because the lake had
dropped to such a low level that the
bottom waters were very oxygenated and
so there were little critters around who
could start moving around in the
sediments and and messing up our nice pretty laminations. Lake Malawi is actually
really cool example because that gray blob 
of sediment that's in the core
on the right that you have there in that handout 
was deposited during what we call  a "megadrought."
This is again a sort of confusing
term, a way that geologists are
confusing everyone. When meteorologists say "megadrought" they mean
the 1930's dust bowl. When we say "megadrought" 
we mean something that
lasted 10 thousand years, or longer. Really, really long.
So this is for lakes. The same thing applies to the marine realm. And marine cores are really fantastic
because they can go back tens of millions 
of years or even further.
Marine mud has been there for a really long
time. And we can take advantage of that.
So we can apply a lot of these same tools, 
and we can apply other tools,
and we can start to look at how the global
carbon cycle has changed, for example, over time.
So this is really where most of our knowledge 
comes from, when we talk about
very long-term changes in climate. Not
short-term like we're seeing today, but
long, long, long-term changes in climate.
