At the Planet Under Pressure conference in London, Diana Liverman and Will Steffen present something of a contrasting couple.
The two professors have been working together on a State of the Planet report, which has involved trawling through numerous reports and scientific papers. At the end of it all, the message of one appears somewhat optimistic, the other fundamentally pessimistic.
They agree that changes to the world since about 1950 have been startling - rapid spread of the human population, accelerating exploitation of forests and marine resources, surging economic growth in successive waves across the world, and so on.
This radical reshaping of the natural world by a single species is certainly unprecedented in Earth history, which a few years back led to scientists Paul Crutzen and Eugene Stoermer coining a special name for our epoch - the Anthropocene.
(The Planet under Pressure organisers have made an animated video of humanity's journey of expansion, by the way, which you can see here.)
If you accept the premise that we have entered the Anthropocene, one of the over-arching questions is "what happens now?"; another is "can we get out of it?"
Prof Liverman, who studies social aspects of global change at the University of Arizona, has the task of assessing the societal trends that either indicate we're heading further into Anthropocene territory or beginning to back out.
Since 2000, she says, some trends have begun to reverse - in particular human fertility, which has halved globally in the past few decades as women have had progressively more access to family planning and maternal health services. In time, this should see the global population stabilise.
There has been a change in food production too.
"In agriculture, the big idea used to be that we destroyed tropical forests [in order to raise food production]," she says.
"What we've seen is that is turning around in some parts of the world where people are growing more food without encroaching on forests - in countries such as Vietnam, the forests are starting to return."
The carbon intensity of industry has reduced too. Companies are finding ways of doing business that are more frugal with energy than before, and saving money in the process.
As a physical scientist, Prof Steffen's role is to see whether these changing human trends are reflected in the condition of the natural world - the oceans, the atmosphere, the land. Overwhelmingly not, he says.
"Over the last decade, with a couple of exceptions we cannot yet see any effect of these trends on the human side. CO2 emissions increased by a larger rate post-2000 than pre-2000 even though we're more efficient - it's just swamped by rising consumption."
The main exception is ozone depletion, which has been arrested, if not yet reversed, by the Montreal Protocol.
Other than that, we appear to be heading deeper and deeper into the Anthropocene. But what does that mean?
No-one really knows. The trends driving global change are unprecedented, so history can give only hints, not a full answer.
Computer models struggle to give precise answers even on single issues, such as climate change or the response of ocean ecosystems to temperature change.
There's a lot of talk about runaway effects. It's said, with some evidence to back it up, that warming and deforestation in the Amazon could combine to create feedbacks that destroy the forest, or that the Greenland ice sheet could begin to melt irreversibly.
Prof Steffen raises another possibility - that the Earth system will stabilise again, but under a different set of conditions, which would be a lot less suitable for the whole range of nature that we find today.
This conference, Planet Under Pressure, has assembled several thousand delegates from academia, business, campaign groups, and the occasional government representative.
It's designed to get people from science and the policy field together three months before the Rio+20 summit in June, to discuss where we are, where we might be going, and how the supertanker workings of our global society can be turned round, if that's what needs doing.
Much of what I've so far read and heard, though, seems very familiar:
- "we need" to adopt a different development model
- "we have to" move to more efficient farming
- "we have to realise that" current western consumption patterns aren't sustainable.
Will one more conference, one more set of reports and - in June - one more global summit bring about these changes?
At the end of Monday's morning session, conference host Nisha Pillai asked the packed hall of delegates for a show of hands on this most basic question - will the changes that "we need" happen?
The noes outvoted the ayes.
Best wishes for a balmy Anthropocene.