Climate change has become one of the dirtiest phrases around which probably explains why reaching the grim climate milestone of 400ppm carbon dioxide has barely rated a mention this week. No-one wants to know bad news! (Although that didn’t save us from this week’s Budget.) So if you’re thinking, ever so reluctantly, the shock jocks and deniers are oddly out of step with the sense of alarm being raised by global experts here is a little primer of what the scientists have been trying to say, to bring you up to speed.

Climate change is, first and foremost, a consequence of the addition of carbon dioxide into the atmosphere. We emit carbon dioxide, through burning fossil fuels or forests, and some of that carbon stays in the atmosphere, intensifying the heat-trapping greenhouse effect and warming the climate. What kind of global warming we’ll see in the future will largely be due to how much carbon dioxide—and other greenhouse gases like methane—we add to the atmosphere. To fully understand the future, we need to understand the present and the past, and track the concentration of CO2 in the Earth’s atmosphere. Concentrations of CO2 in the atmosphere have been tracked at in Hawaii since 1958 when levels were just under 380 ppm. Plotted on a graph, the readings over time curve upwards sharply as humans added more and more CO2 to the atmosphere.

The National Oceanic and Atmospheric Administration says the amount of heat-trapping carbon dioxide in the atmosphere is about to top 400 parts per million The last time CO2 levels were this high was likely during the Pliocene epoch, between 3.2 million and 5 million years ago. The Earth’s climate was warmer during the Pliocene than it is today—perhaps by 2 to 3 C—and sea levels were much higher. It was a very different planet than the one we’ve lived on so successfully for thousands of years.

The 400 ppm threshold has been an important marker in U.N. climate change negotiations, widely recognized as a dangerous level that could drastically worsen human-caused global warming. If that’s not cause enough for alarm the sheer rate of increase over just the past 55 years shows how fast global warming could hit us—and underscores how much more we need to do as a planet to slow down carbon emissions.

The U.N.’s official goal is to keep CO2 levels below 450 ppm and we’re rapidly running out of time to make that happen. CO2 can stay in the atmosphere for centuries, which means that we’ve already baked in far more warming than we’ve yet experienced and set our planet on a trajectory for a hotter and wilder future.

Since the introduction of the carbon tax Australia reduced its carbon emissions from electricity sector with our seven most emissions intensive generators producing around eight million tonnes less carbon emissions than for the same period last year. This is the first sign that we can curb our pollution, even in coal loving Australia.

Tracey Carpenter is President of Bathurst Community Climate Action Network