『Abstract
The carbon cycle binds together earth's ecosystems and their
inhabitants. My intent is to review the global carbon cycle, examine
how humans have modified it, and contemplate (from a soil science
bias) the new questions that await us on a changing earth. These
thoughts are proffered, not to propose a way forward, but to invite
conversation about opportunities that await us.
Terrestrial ecosystems hold a lot of carbon - about 500 Pg C
in plant biomass, and 2000 Pg C in soil organic matter. Oceans
contain even more. And the atmosphere, now with about 785 Pg C,
connects all of these pools. The flows of carbon between the pools,
and their feedbacks, have kept atmospheric CO2
reasonably constant for millennia. But humans have increasingly
distorted the balance, by changing land use and by injecting fossil
C back into the cycle. Consequently, atmospheric CO2
has increased recently by more than 3 Pg C per year and, by century's
end, its concentration may be twice pre-industrial levels, or
more.
The changing carbon cycle poses new questions for scientists.
Now we will be asked, not how things are, but how they will be.
For example: How will changes in CO2 alter
flows of carbon through biological carbon stocks? Can we manage
ecosystems to hold more carbon? Are current carbon stores vulnerable
should the earth warm, or water cycles shift, or nitrogen flow
be altered? What will the C cycle look like a century from now;
and will it then still provide all that we expect from it? These
and other new questions may elicit from us fresh insights and
approaches. We may learn to look more broadly at the C cycle,
seeing all the ‘ecosystem services’ (not just C sequestration).
We may insist on studies yielding deeper understanding of the
C cycle, relevant beyond current issues. We may further emphasize
‘time’ in our studies, looking more at flows and changes than
at describing what is - and looking long enough to see even subtle
shifts. We may learn to follow C beyond the usual boundaries set
by arbitrary disciplines. And we may come to see, more than before,
how the carbon cycle weaves through our fields and skies and forests
- and find new ways to reveal its grandeur to those who have not
yet seen it. And then, it may happen that our successors, a century
from now, will look back, almost in envy, at the urgent, enticing
questions we were given to solve.
Keywords: Carbon cycling; Greenhouse gases; Climate change; Global
change; Carbon; Nitrogen; Carbon sequestration; Mitigation; Land
use; Fossil fuel』
1. Introduction
2. The global C cycle
3. Human influences on the global C cycle
3.1. Land use change
3.2. Fossil fuel burning
3.3. Coming changes
4. Challenges and opportunities on a changing earth
4.1. Consider all ecosystem functions
4.2. Aim for fundamental understanding, pertinent beyond the
‘problem of the day’
4.3. Consider time - explicitly
4.4. Follow C beyond usual boundaries
4.5. Admit (and quantify) uncertainty
4.6. Relish and reveal the C cycle's beauty
5. Closing thoughts
Acknowledgements
References