Climate change is a like a runaway train. It has picked up a lot of momentum and even after you hit the brakes global carbon emissions will take a long time to slow down. There may even be some particularly belligerent passengers that want you to pick up more speed.

Until recently the international consensus was that we needed to stop before reaching 2°C. There were even discussions that we may have to travel past this stopping point and go into reverse, by taking carbon dioxide back out of the atmosphere.

Then at COP21 in Paris last year there was an unexpectedly well-received response to the call to raise our collective ambition. It was agreed that the world would set the goal of keeping global temperature increase to well below 2°C, with efforts to limit warming to no more than 1.5°C. This lower threshold would significantly reduce a number of the serious risks associated with climate change.

This ambition is now enshrined into international law through the Paris Agreement, which came into force earlier this month. But despite the fact that it is a laudable goal, there are still huge questions over whether it is practically and politically possible. This will be the focus of many of the discussions taking place in Marrakech at COP22 and over the coming months.

2016 is already going to be the warmest year on record, with temperatures at least 1°C above the pre-industrial average. There isn’t a lot of room left for manoeuver. On current trends the first years that we experience a taste of 1.5°C of warming could be just a decade away.

According to the UN Environment Programme we are currently on a trajectory for somewhere between 2.9 to 3.4°C of warming by the end of the century, even if every country follows through with all of their pledges.

The only hope is to go a lot further and faster than current plans allow. But if we do not significantly increase action in the next few years then keeping the average temperature rise below 1.5°C goes from being a bold hope to a pipe dream. Even the most optimistic scenarios will require massive reductions, some overshoot and a substantial deployment of negative emissions options.

So what do we need to do to get there? This is where things get difficult. There’s not a lot to go on, in part because it was a big surprise. Given the previous positions from policy makers and businesses, forecasts were pragmatic rather than optimistic. Scientists and economists had focused their efforts on exploring pathways to 2°C and above, and leading companies have been setting their own targets based on those trajectories.

To date there has only been limited research on what is required under a scenario where we stabilise warming at 1.5°C. This means there is a lot of frantic work going on behind the scenes to provide the evidence base for action, with the centrepiece expected to be the special report on the topic by the Intergovernmental Panel on Climate Change which will be released in 2018.

The challenge is that we already know the window for action is only open for a few more years. There is no time to delay, or to wait for a clearer picture. We need to start acting and we need to do it now.

At the moment governments need to collectively deliver an additional 25% of reductions beyond current pledges by 2030 to get us on track for 2°C. But getting to 1.5°C means going beyond even this, probably ensuring that global emissions peak before 2020 and then start dropping off steeply.

One thing that we can figure out is that the pathway to 1.5°C looks something like the one to 2°C, but with a more of every solution implemented sooner. We need more low carbon energy, better energy efficiency, a quicker shift to sustainable transport and a revolution in green agriculture. We will almost certainly need a lot more deployment of negative emissions later in the century, both through natural carbon sequestration and novel technologies to remove emissions from the atmosphere.

Getting there is going to require a lot of commitment and an element of risk-taking. We need to be willing to try lots of things out, making the best of whatever innovation and behaviour change is possible, rather than waiting for a silver bullet that may never arrive.

But there are reasons to be cautiously optimistic. After the unexpected successes of Paris last year – followed by one of the quickest ever ratifications of an international agreement – we have recently also seen a global agreement to eliminate the use of hydrofluorocarbons, a particularly potent greenhouse gas, and the aviation industry has struck a landmark deal on emissions.

The important thing now is to do whatever we can to slam the brakes on the runaway train as firmly as possible, stop it from derailing and find a way to switch it into reverse gear. Then maybe we can stop at a far less dangerous final destination.