Flash back to 2011 and the arguments against such action were loud and plentiful. The most common argument was that Canberra's emissions were tiny compared to the global total; so small that they were negligible. Reducing our emissions would not affect the global temperature at all, so why should we bother, especially when no one else was reducing their emissions?

This argument is still with us. We hear it all too often at the national level: Australia's emissions are too small to matter. China and the United States are the big problem. And we hear it at the company or business level. Why bother when our emissions are so tiny?

At first glance, the argument seems very appealing. Canberra's emissions are indeed very small. At about four million tonnes of carbon dioxide in 2011, they were about 0.012 per cent of the global total. Absolutely miniscule.

But this argument is a fundamental fallacy when dealing with collective-action problems, where millions of individual players, each contributing a very small amount to a problem, add up to create a very big problem. Climate change is a collective-action problem par excellence.

This fundamental fallacy was recognised by Chief Justice Brian Preston, of the NSW Land and Environment Court, when he ruled against the proposed Rocky Hill coal mine. He noted that greenhouse-gas emissions are made up of millions, and probably hundreds of millions, of individual emissions around the globe. The problem of climate change needs to be addressed by multiple local actions to eliminate emissions at their source.

If you are still unconvinced, try the "too-small-to-matter" argument on the Australian Taxation Office. For the vast majority of Australians, our income tax is so small that it is negligible compared to the Australian government's overall revenue. But I suspect that if you write to the Tax Office to say you will no longer pay your income tax because it is so small, you will not get a positive reply.

So how do we know what Canberra's fair share of the global emission-reduction challenge actually is? Scientists often use the carbon-budget approach, which estimates the allowable global emissions to have a good chance of meeting a given temperature target - say, the Paris targets of well below 2 degrees and aiming for 1.5 degrees, compared to pre-industrial temperature.

From the beginning of 2019, the global carbon budget to cap temperature rise to no more than 2 degrees was about 515 billion tonnes of carbon dioxide. On an equal per capita basis, Canberra's share of that budget would be about 30 million tonnes of carbon dioxide.

That sounds like a lot, but our 2011 emission rate of about four million tonnes a year would have left only seven or eight years before we must have completely decarbonised.

So how are we tracking now?

In 2011, Canberra's annual emissions were about nine tonnes of carbon dioxide per person, compared to the average for Australia of about 17 tonnes per person. We've already made a huge step forward by reducing our emissions by more than 50 per cent compared to 2011 in just over one decade (40 per cent reduction compared to 1990). This remarkable outcome will be achieved in the early 2020s when we are sourcing 100 per cent of our electricity from renewables. Then our annual per capita emissions will drop to about 4.5 tonnes of carbon dioxide.

Building on that success, the ACT government recently legislated to achieve net zero emissions by 2045, with interim targets for 2025, 2030 and 2040. Each year has an upper and lower target range.

The total ACT emissions from now until net zero in 2045 would be 28 million tonnes of carbon dioxide for the upper target range and 23 million tonnes for the lower target range. Both are below the emissions level for the 2-degree Paris target.

As well as doing our fair share to tackle climate change, Canberra's actions have yielded other co-benefits. The reverse-auction process has helped to minimise the extra cost of moving to 100 per cent renewables. Canberra's leadership kept the large-scale renewable-energy industry alive in Australia when federal policy turned against it.

The transition to renewables brought new businesses and jobs to Canberra. For example, two global wind-power companies moved their headquarters to Canberra, and training schemes for the installation and maintenance of renewable systems were established in the city.

Canberra is now recognised as a climate leader nationally and globally, showing how effective action to reduce emissions can be achieved while building a growing and vibrant community.

But the biggest benefit of all will come in 2045 when we can look our children and grandchildren in the eye and say: "We've done the right thing for you."