That's about the same magnitude of change that occurred from the last ice age to today, over a timeframe at least 10 times shorter.
My view essentially shared by the World Bank, among many others is that society adapting to that radically changing world is probably impossible. That's a polite way of saying we'd be screwed.
The current pledges countries have taken to Paris lower the projected warming to around 3C, according to the independent scientists at Climate Action Tracker.
Progress has been made, but it's not enough.
Three degrees would still carry dire impacts for human health, nutrition and security; multi-metre sea level rise would almost certainly be locked in; and the odds of abrupt, catastrophic changes would be unacceptably high.
With faster and deeper emissions cuts, experts say that keeping warming to less than two degrees - perhaps even 1.5C is still within our grasp.
Two degrees is no magic number really but it's an important signpost of the risk being passed on.
It will still pose major adaptation challenges and likely lead to low-lying Pacific Island nations becoming uninhabitable.
But the world has agreed it should be our upper limit, and this gives hope for a decent future for today's youth.
All of that is why UNICEF has declared that inaction on climate change is a violation of child rights.
That's the gloomy stuff. The flipside is that decisive action presents a great opportunity to build a brighter future for youth.
Climate action is an investment, rather than a cost.
The transition will create millions of decent jobs building clean energy and transport systems.
It's an opportunity to make our cities better and improve people's lives.
Young people today want more than just a pay cheque - they're increasingly looking for purposeful work solving problems that matter.
Building a zero carbon world can be our generation's great purpose.
I think there is so much potential for innovation and creativity waiting to be tapped.
Q. What is the level of awareness among our young people of this issue and how much do you feel they know about the specific impacts of climate change?
It's certainly growing.
A recent study by Motu Public Policy found that under-25s were much more likely to accept the science of man-made climate change and see it as a threat to New Zealand than older age brackets were.
But I do feel there is still quite a bit of confusion and doubt, and lack of knowledge about specific impacts, which we need to address.
I think for too long the topic has been pigeon-holed as an "environmental" issue - the endless pictures of polar bears don't help which has made it more challenging to communicate the human dimensions.
For example, I doubt very many people would be aware that the US Pentagon has declared climate change a major risk to national security.
Q. In New Zealand, how have youth been making themselves part of the discussion and the decision-making process?
With the increasing numbers of conversations being had online, that's one outlet for youth to engage in discussions on issues like climate change.
A problem with that is it's generally less visible to decision-makers than, say, the letters page in the newspaper.
Frankly, trying to engage in the decision-making process has been hard.
Especially at the national level, where there just hasn't been any real effort to reach out and ignite the national conversation about climate action.
That hasn't stopped some youth from trying, though.
With Generation Zero, we've done a lot to try to raise awareness of opportunities to affect relevant local and national decisions, and make it easy for like-minded people to have their say, using tools such as quick online submission forms.
Many young people have joined up and volunteered with us to try to have greater impact on the decisions that affect their future.
Q. Are they being listened to and taken seriously?
You'd have to ask the politicians.
From Generation Zero's perspective we do feel we've been listened to at the local government level and had some fairly strong influence there.
We also get a lot of approaches from other organisations and businesses who really want to hear from young people.
We were disappointed that all the submissions to the Government regarding its climate targets earlier in the year seemed to fall on deaf ears.
We put a lot of work into that and there were thousands of passionate, articulate submissions from people young and old.
On a more personal level, I had a conversation with a friend recently who was really upset after having a conversation about climate change with a couple of older people she knew, who had been pretty dismissive and patronising.
It's really important to hear young people's concerns.
Q. Have you found many of our youth are disinterested or turned off by climate change, if just because of the scale and complexity of the issue?
Yes, some are.
In many ways climate change is like a perfectly designed problem to foil human psychology - there's a generational time lag between our actions and the consequences, the causation is invisible and indirect, there's no one clear enemy - well, Exxon Mobil seems to be trying pretty hard to change that.
When you think about it, it's kind of amazing that there is the level of interest that there is.
Q. Why was Generation Zero established and how has it been representing youth in this area?
The seeds were planted with a small group of us who went to the UN climate summit back in 2010 in Cancun, Mexico.
We realised through that experience that the necessary political action was only going to come from citizens making their own governments do it.
We believed and still do today that New Zealand has the potential to lead the way to a zero carbon future.
We wanted to create a strong voice for the young generation that needs to make that transition happen over our lifetimes Generation Zero.
We've grown a lot since we started and have built a sort of national community of young people who care and are taking action in various ways, as well as a wider network of supporters.
Our activities have included running campaigns at the national and local level, developing and communicating solutions and policy ideas, and speaking up to inject a youth perspective into the discussion about climate change and related issues such as transport.
Q. What specific actions has the group proposed and how have these been received so far?
One of the bigger ideas we have pushed is for a New Zealand equivalent of the UK Climate Change Act - a "carbon budgeting" law.
This would establish an independent climate commission, set emissions reduction targets into the future, and establish a legislated process to ensure that governments actually develop a clear plan to meet those targets.
It's the sort of overall framework that New Zealand needs, because our current approach is a mess.
National have said they don't want to do it but Labour, the Greens and New Zealand First back the idea, as do a number of other organisations.
A big area of focus has been on transport and urban development.
Transport accounts for roughly 40 per cent of New Zealand's carbon dioxide emissions and we think there are big gains to be made here while delivering a lot of other benefits too.
We teamed up with Transportblog to develop the Congestion Free Network - a comprehensive plan for a world-class public transport network for Auckland, which has garnered major support.
We've also campaigned for "density done well" compact urban form with more housing choices and lower carbon emissions.
We've done similar work in other main centres, as well as pushing for more funding for public transport, walking and cycling from local and central government.
On the latter there has been success with the Government's urban cycleways funding programme, which is fantastic.
Q. Do you feel the key to bringing about climate action lies in grassroots movements like this one?
Personally I don't actually think there is one key to unlock the action we need.
We need people working on all facets of the problem in different ways, because ultimately we don't know what will make the critical difference.
I suspect we need pretty much everything working in tandem: policy and institutional change, technology development, social change the lot.
That said, I do believe movements have a critical role to play, as they have in achieving social progress throughout history.
We do need government action, and rather than hope the politicians will do it, we need to make them do it.
As former Prime Minister Sir Geoffrey Palmer said recently on this, "you need the citizenry making a lot of noise".
I'm filled with hope and inspiration following the People's Climate Marches that took place last weekend around New Zealand and the world.
Over 35,000 people marched throughout our country, and it was the coming together of a huge range of groups including faith groups, health organisations, unions and more.
It really feels now like we're on the cusp of a social tipping point.
Q. What is the global picture of youth involvement in the climate issue: are we seeing good enough representation of our young people around the world?
One pretty awesome thing which the internet has really helped is there is a global network of youth climate change activists who support one another.
It's great knowing that there are heaps of people like you trying to do the same thing in their countries, to stay hopeful.
I can't speak for other countries but I would say globally there is a lack of youth representation in the negotiations.
I'm reminded of a conversation I had with Nick Smith, who was Climate Change Minister at the time, in Cancun back in 2010, when he said that the negotiations were essentially all about "who is going to pay".
The intergenerational problem is that by collectively failing to do enough, current leaders are deciding that younger generations will pay but younger generations don't have a seat at the table.
Q. What presence do you expect youth to have at the COP21 talks in Paris, and what optimism do you have of getting a meaningful outcome from the negotiations?
Several friends and Generation Zero members are over in Paris now as observers, and they've already made their presence known with a fair bit of media attention.
They'll be watching our government's actions closely.
Together with others from around the world trying to create an atmosphere where the negotiators and world leaders remember what is at stake here and strive for the best agreement they can get.
It's clear Paris will not yield the ideal outcome we want because countries are not putting ambitious enough pledges on the table, so we need to accept that.
But it could still deliver meaningful progress as the first climate agreement with all countries committing to reduce emissions.
I hope we will see agreement on: an ambitious global goal of achieving zero net emissions as early as possible, mechanisms to ratchet up ambition and hold countries accountable, and greater commitments from wealthy countries to help poorer countries develop cleanly.
Whatever happens, the commitments governments have made need to be seen as a floor rather than a ceiling.
••Paul Young will be joining a panel on the Paris climate talks organised by the New Zealand Institute of International Affairs (NZIIA) and to be held at Victoria University tomorrow (Friday) morning.
Other panelists include Maty Nikkhou O'Brien and Sir Douglas Kidd of the NZIIA, Victoria University scientists Professor Tim Naish and Dr Jim Renwick, Motu Economic and Public Policy Research fellow Catherine Leining and Herald economics columnist Brian Fallow.
People wishing to attend the panel discussion, running from 8am to 9am at Lecture Theatre 1, in the Old Government Buildings at Victoria's Pipitea Campus in Lambton Quay, should register at nziiaclimate.eventbrite.co.nz.
