The defining choice facing leaders in Glasgow this week at Cop26 is whether to sugar-coat reality or be honest about the climate emergency, and demand the action that will be necessary to confront it. If we are to have any chance of preventing catastrophe, we must choose truth and candour.

The most important truth is the maths. For all the millions of words spilled about this summit, not enough has been done to spell out its central task. Many leaders say we need to limit global warming to 1.5 degrees, but few say out loud what that means.

We know from the UN’s figures that after the Paris accord of 2015, the world was heading for something like 53 billion tonnes of emissions in 2030. To keep global heating in line with 1.5C degrees, we need to cut emissions to 25 billion tonnes by then. There is an emissions gap of 28bn tonnes to be cut in this decisive decade. These numbers should be in the minds of everyone who cares about our planet’s future.

The world has taken important strides forward in recent years. But the unfortunate reality is that the pledges made in advance of Cop26 amount to a cut of just 4bn tonnes. This leaves an aching emissions gap within which climate catastrophe would occur – putting us on track to a devastating 2.7C. This maths matters more than anything, and should shape our response to what countries announce at Glasgow. According to the respected Climate Action Tracker, no G20 country is doing enough.

It will be in the interests of leaders and ministers at Cop26 to pretend more progress has been made than is the reality. Since we cannot negotiate with the science, we must force them – particularly those who have arrived in Glasgow only to reheat old commitments – to confront the truth and negotiate with each other. This summit cannot be a fortnight’s fiesta of pre-packaged announcements; it must amount to a real reckoning to deliver the progress we need.

As part of this, leaders must deliver justice for those in poorer countries who are least responsible for, and most vulnerable to, climate breakdown. It’s time to make good on the promises stacked up, but not delivered – in particular the long overdue $100bn of finance that was promised for developing countries back in 2009, and the prime minister’s pledge at the G7 in June to “vaccinate the world” against Covid. It’s essential that we also recognise the loss and damage many poorer and developing countries face from climate breakdown. This is the way to help reassemble the coalition we saw in Paris between the most vulnerable developing countries and ambitious developed countries, to maximise pressure on the world’s biggest emitters, including China.

We must also head off the attempt to shift the scientific goalposts at Glasgow, which has crept into the UK government’s benchmark of success for these talks. It is positive that countries representing 80% of global GDP are now covered by a net zero target for the middle of the century, but targets three decades in the future do not make up for a failure to act now.

No country can be allowed to think that setting long-term targets gives them an alibi to not act in the short term. Take Australia, for example, whose 2050 net zero target, announced last week, is accompanied by a 2030 target which puts us on track for a 4C world. Or Saudi Arabia, which is mulling a 2060 target, while planning to increase its oil production over this decade. We cannot stand for such global greenwashing at this summit from our country or anyone else’s.

Telling these truths is not without its risks. Some people will worry that any admission of falling short in Glasgow will give the world and private investors reason to give up on their climate ambitions. Many of us remember the long shadow that was cast after the 2009 Cop in Copenhagen, when the summit crash landed without comprehensive agreement. We cannot afford the same hangover, whatever the result in Glasgow. The world has come too far for that to happen. But that cannot be reason to pretend progress is fast enough if the reality is otherwise.

That’s because the inescapable truth is that, with the climate crisis, unlike so many other issues, winning slowly is not really winning. In this existential race against time, winning slowly means billions of people facing extreme heatwaves, countless millions more people displaced, and the destruction of natural wonders like our coral reefs. It also means forfeiting the opportunities that climate action offers, such as lifting people out of poverty by creating millions of good, green jobs.

The truth matters both for the outcomes of Cop26 but also for what happens afterwards. The world is not due to return to the issue of country-by-country pledges until the next Cop26 happens in 2025. If we follow this course, we will forfeit 1.5C. We will need to come back much sooner to close the gap we face if we are to create a path out of Glasgow compatible with 1.5 degrees.

We are all desperate for success over the next fortnight. But as we approach this crucial summit, those of us who are not in government have one power those in office will find it hard to fully embrace: the power to tell the truth about our progress in this decisive decade, however uncomfortable that may be.

Playing our part in getting the right outcome means using this power without hesitation in the next two vital weeks.