Now, feedback processes are divided into two: positive feedback and negative feedback. (negative) Let's take that deer-and-grass example
again. When the amount of grass decreases, less grass leads to less deer — a bunch
of deer die. Positive or negative? What do you think? Huh? Well I'm prepared to bet
that many of you are thinking, "That's negative," — decrease in the number
of deer — minus sign in a population of deer — negative. Or alternatively,
"poor Bambi" — emotionally negative. Forget that thinking. It has nothing to do with this concept here. Positive feedback is when the cause and the effect
go in the same direction: an increase leads to an increase, a decrease leads to a decrease.
Negative feedback is when the cause and effect go in the opposite direction: an increase in
something leads to a decrease in something else; a decrease in something leads to an
increase in something else. OK? So less grass leading to less deer — a decrease
leading to a decrease — is positive feedback. The decreased number of deer allow
more grass to grow — decrease leading to an increase — opposite
direction — negative feedback. The increased grass allows an increased number
of deer — increase leads to increase — positive. Right? And then the increased number of
deer lead to a decrease in the amount of grass — negative. OK? So maintaining that
balance — that zigzag — basically involves alternating positive, negative, positive,
negative, positive, negative, feedback. And the negative feedback is the most important
one of the two, because when things start to move away from equilibrium, negative feedback is what
brings it back — keeps the equilibrium stable. Negative feedback keeps a system in balance. Now let's consider an example where we have
positive feedback and no negative feedback. What happens? The example I'm going to give
you now is — we'll take the whole planet, OK — the water cycle. Water evaporates,
goes into the sky, forms clouds, and precipitation. Precipitation over
most of planet Earth falls as rain, but there are some places where it falls as
snow. Near the north pole, near the south pole, where the temperature is cold, it falls as
snow. Sorry this illustration's a bit cheesy, but it does show the snowy regions around
the north pole and around the south pole. Snow is white. White intercepts the heat coming
from the sun and blasts it back into space, keeps the earth cool, OK? So, let's start this system with a process .
. . with some external process that cools the planet down a bit. What happens? The area of
snow around the north pole and the south pole gets bigger. Bigger area of the earth becomes
white — repels more of the sun's heat — the earth gets colder. That in turn
makes the area of snow even bigger, which repels yet more of the sun's
heat, and the earth gets colder yet, which expands the area of snow
further . . . You get the picture. Within an incredibly short space
of time — years or decades — we've gone from a warm climate
like today, into a full all-on ice age. Now again, don't get confused by Hollywood.
There's a movie called The Day After Tomorrow, where they managed to get to an ice
age over the course of a long weekend. That's Hollywood: that's utterly absurd. But you know, for the longest time — and
some people still today — think that it takes thousands or millions of years to get from
a warm climate to an ice age. No, no, no! A few tens of years at most. Positive feedback.
Once the process kicks in there's no stopping. It gets colder, colder, colder, colder, colder.
Totally happened within a person's lifetime. And we've got geological data that
shows that, repeatedly, in the past. How do you ever get out of this? Now the Earth
is an ice cube, it's white, it repels the heat from the sun, it's going to stay cold, isn't
it? Well, to break out of that, we need a new process, something different to kick in. The
feedback processes we've been talking about, which were positive only — going in the same
direction — there's no . . . there's no breakout. This brings in the important concept of a threshold. A
threshold is when something different kicks in, and breaks the cycle. In our ice age example, how does that work? Well,
the ground — the land — cools down pretty quickly. The ocean is not just broad, it's
also very deep; the water contains a huge amount of heat. It's going to
take a long time to cool the ocean down. That's going to take thousands of years. Gradually the ocean temperature drops,
drops, drops. Once the ocean is cold, what happens to the rate of
evaporation? Evaporation drops off, and now the question, "Is precipitation going to
fall as snow or as rain?" doesn't really matter, because if there wasn't any evaporation, there
is no precipitation of either rain or snow. So now we stop adding white. The Earth is frozen: doesn't it stay
white? Well, any of you who've lived somewhere where it stays cold throughout
the winter will know the answer to that one. Over time snow gradually disappears. We always
think of snow melting and turning into water, but actually, under the influence of
sunlight, very slowly, snow evaporates — it goes straight from solid to gas — can bypass
the liquid. So at temperatures below freezing, the amount of snow on the ground very slowly
decreases, decreases, decreases. Eventually as the snow starts to disappear, the ground turns
from white back to dark. The dark absorbs the sun's energy, the planet heats up, the ice
age is over, almost as rapidly as it began. OK? So hopefully these illustrate the concepts
of positive and negative feedback and thresholds. Another example in your book for a threshold
— very simple one — imagine you have a pot plant; you water it regularly. One day you give it some
fertilizer and notice that it grows better. So you give it a little more
fertilizer, and it grows even better. Then you overdose it on fertilizer — and kill it!
That's the threshold. That's the book example. I like the ice age one better
personally but, anyway, you get the idea.