Over the past two years in the US, concerned citizens have been galvanised to march, rally, campaign and get arrested to block the Keystone XL tar sands pipeline – a project to pump a reported 830,000 tonnes of one of the world's dirtiest fossil fuels from Canada to Texas.

The decision is still sitting with Barack Obama. Environment group the Natural Resources Defense Council says blocking the project will prevent as much as 24.3m tonnes of CO2-equivalent being released every year.

Over the 50-year life of the project, that's 1.2bn tonnes of greenhouse gases – a huge amount.

Yet if this level of emissions seems irresponsibly high – which it surely is in a carbon-constrained world trying to avert the risk of dangerous climate change – then how should we categorise 3.7bn tonnes of CO2-e, a figure more than triple that from the Keystone XL proposal?

That 3.7bn tonnes is the total emissions of CO2-e which could be emitted by just two linked mega coalmines in the Galilee basin in Queensland, Australia, which have both been approved for development.

The first mine is part of the Alpha coal project, approved in August 2012, and is owned by Indian energy conglomerate GVK, which bought the coal interests from Australia's richest person, Gina Rinehart (who doesn't accept the science of human-caused climate change), in September 2011 as part of a $1.2bn deal.

In the months leading up to the sale, Rinehart flew three federal MPs to India on her private jet to attend the wedding of Mallika Reddy, granddaughter of GVK's founder GV Krishna Reddy.

Farmers and environmentalists have challenged the Alpha mine's approval in the Queensland land court, saying it will put vital underground water at risk and contribute to climate change. A decision isn't expected until early next year.

Rinehart's Hancock Prospecting company still retains a 19% stake in the second mine – known as Kevin's Corner – which is majority owned by GVK and was approved by the environment minister, Greg Hunt, this month. Another court challenge is being considered.

Climate change campaigners in Queensland are also trying to pressure rail company Aurizon to pull out of an as yet unsigned deal to develop the railway line needed to get the coal from the Galilee mines out to Abbot Point, where an expansion of the coalport is being planned.

So where do I get that 3.7bn tonne figure come from?

During the case in Queensland's land court, Hancock Coal appointed an expert to submit a report outlining the emissions from the Alpha coalmine – accounting for all the mining operations, releases of methane from the ground, use of explosives, transport, shipping and the eventual burning of the coal in power plants.

The report, which was submitted to the court and which I have seen, indicates the Alpha mine will emit an average of 61.9m tonnes of CO2-e every year for the 30-year life of the project.

If you add up the different categories of emissions detailed in the report, over the course of 30 years the project emits 1,857 Mt of CO2-e. This, the report detailed, was based on a total of 839.6 Mt of processed coal being produced and exported.

Now to the Kevin's Corner project, which it is proposed will use an extension of the rail line being proposed from the Alpha mine to get the coal to Abbot Point.

According to the environmental impact statement submitted by GVK, Kevin's Corner would emit an average of 2.02 Mt of CO2-e a year for a mine with a 30-year life.

That's a total for the life of the mine of 58.57 Mt of CO2-e. But this figure doesn't include the emissions from burning all that coal, because there's no requirement in the law to provide this, just as environmental approvals don't take into account the impact of projects on climate change.

But the report does say that the mine will produce an expected 856 Mt of coal ready for export, a similar amount to the Alpha mine.

For a conservative estimate of the emissions from burning the coal, we can take the emissions from transportation and burning coal from the Alpha mine as a proxy for the neighbouring Kevin's Corner project. This figure is 61 Mt CO2-e per year, or a total of 1,829 Mt over the 30 years. Add this to the other emissions from Kevin's Corner, and we get a grand total of 63 Mt of CO2-e per year or a total of 1,887 Mt Co2-e.

So, the total contribution of these two proposed coalmines in the Galilee basin will be 124 Mt CO2-e a year for 30 years or a whopping (I think this adjective is now allowed) 3,745 Mt Co2-e. That's 3.7bn tonnes of CO2-e.

Yet the vast bulk of those emissions will not make it to Australia's emissions accounts, because the coal will be exported and burned, mainly in India. The climate, however, couldn't give a monkey's where the emissions come from. The impact is still the same.

But what does this massive carbon footprint mean in context?

For one thing, these two mines' annual contribution would practically scrub out the savings from Australia's emissions reduction target.

By the year 2020, the Australian government has committed to cutting the country's emissions by 5% from the levels they were at in 2000. That means that emissions in 2020 will be 159 Mt lower than they would have been if there was no target.

A Greenpeace report has calculated that if the Alpha coal project was a country, its annual emissions would be higher than the likes of Austria, Columbia and Qatar.

The United Kingdom's carbon footprint gives us another comparison. The UK emitted 571.6 Mt of CO2-e last year. The total contribution of the two Galilee basin mines – if stopped – would be like making Britain carbon neutral for six years.

Yet these two megamines might just be the very large tip of a melting iceberg for the Galilee basin.

The Queensland government has today announced a strategy to speed up coalmining projects in the Galilee basin, giving "first movers" a discount on the royalties they will pay for mining the coal as well as making approvals easier.

The Indian-owned Adani Mining also has plans awaiting approval for a Galilee mine which, at its peak, would be producing 60 Mt of coal a year – double the capacity of the Alpha project.

Newly elected federal MP Clive Palmer's China First coal project also wants to mine about 40 Mt of coal a year from the Galilee basin, with the project now awaiting approval by environment minister Greg Hunt.

The latest Intergovernmental Panel on Climate Change report has outlined the world's carbon budget - that is, how much CO2 humans can emit from burning coal, oil and the like before global warming passes 2C. The IPCC took the period between 1860 and 1880 as the time when we started to eat away at the budget.

To give the world a 50:50 chance of staying below 2C, total human emissions of carbon dioxide would need to stay below 840 gigatonnes of carbon .

We have already "spent" 531 GtC of that budget, leaving 319 GtC left to go at.

In terms of carbon dioxide, 319 GtC is 1,171 gigatonnes of CO2 – or 1,171,000 Mt of carbon dioxide left in the budget.

If just these two Galilee basin coalmine projects go ahead, we get that down to 1,167,255 Mt of CO2.