Have you got climate zombies?

They might pop up in your social media feed, or manifest in comments under climate change news online. They might even appear at your Christmas lunch. And they're rife in some media outlets — they often come out after dark.

They're the cases against climate science that were buried years ago, yet somehow, refuse to die.

"It's the Milankovitch cycles", "CO2 is good for plants anyway", "What have the scientists ever done for us?"

You might have even been infected yourself. It's no surprise really, a multi-billion dollar campaign funded by the likes of Donors Trust, Donors Capital Fund, Koch-affiliated foundations, and parts of the fossil fuel industry has been animating these damned corpses since the '80s.

So consider this your own handy guide for killing the undead. Or at least a shield for deflecting their relentless pursuit of brains.

Good luck out there.

CO2 doesn't cause warming

We may as well start at the top, with this wretched creature — the CO2-doesn't-cause-warming zombie.

Your defence? Yes. Yes it does.

The first scientist to show the link between CO2 and warming was Eunice Foote — an American scientist, inventor and women's rights activist.

Her research — Affecting the Heat of the Sun's Rays — published in 1856, is now credited with first identifying the greenhouse effect.

She identified the warming capacity of CO2, as well as "common air" and "damp air".

But her contribution to science was largely written out of the history books after Irish physicist and atmospheric scientist John Tyndall made a similar finding in 1859.

He found that water vapour, carbon dioxide and ozone were very efficient in the trapping of atmospheric heat.

Then in 1896, another man with facial hair and a science degree called Svante Arrhenius showed just how much warming we could expect from burning fossil fuels.

But CO2 was below 300 parts per million at the time, and his findings were more of a scientific interest than cause for concern.

John Tyndall got most of the credit, but Eunice Foote figured out CO2 caused warming in 1856. ( On the Shoulders of Giants: Roger Kammerer )

Now, more than 100 years later, there's been plenty of research into CO2, but as far as its effect on atmospheric warming goes not much has changed, according to Mark Howden, director of the Climate Change Institute and a vice chair of the IPCC.

"Anyone who says that this doesn't happen has to catch up with 150 years worth of science," Professor Howden said.

"The argument that small concentrations don't matter is an absolutely ridiculous one.

"You wouldn't propose that anyone take half a teaspoon of potassium cyanide — it's a small concentration in your body but we know it's fatal."

"[And] the scale of the increase [of CO2] is huge. In my lifetime it's gone up 100 parts per million."

And while we're on the topic, volcanoes — sometimes blamed for releasing more CO2 than humans — release only about 1 per cent of the CO2 that we do.

It's Milankovitch cycles

As a general rule, if it's something taught in elementary earth sciences, scientists have well and truly thought of it, studied it and discounted it.

While throwing around the term Milankovitch cycles may sound good at parties, it's a pretty simple and flawed explanation for the climate change we're experiencing right now.

Since the 1920s, thanks to Serbian scientist Milutin Milankovitch, we've known that cyclical variations in Earth's orbit and tilts and wobbles of our axis influence climate and drive ice ages and interglacial periods.

The problem is, these cycles wax and wane over thousands to hundreds of thousands of years, and don't correlate with today's rapid temperature increase.

Right now, Earth's tilt is in the decreasing phase, which means the northern hemisphere, where the majority of our landmass is located, is slightly less tilted towards the Sun.

So, if Milankovitch cycles were the dominant driver or "forcing" on our planet, we'd be in a cooler phase where temperatures should be slightly down on average.

There's been a 5? 10? 20? year pause in warming

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This little doozy has gone a bit quiet in the past few years, though it ate quite a few brains in the early 2000s and again in 2012/13.

Thanks in part to a savage El Nino, 1998 was an exceptionally hot year globally — at the time, the hottest on modern record at 0.62 degrees Celsius above pre-industrial levels.

But warming isn't linear, it goes up and down a bit, and so inevitably the following years were a bit cooler.

The myth of the pause or warming hiatus started here.

If you take 1998 as year one, and then look forward to 2002, it appears that warming has stopped or is trending slightly downwards.

If you ignore the black trend line, it looks like there was a 'warming pause' in the early 2000s. ( Supplied: NASA )

But climate is measured at a minimum of 30-year periods, according to Will Steffen and Annika Dean of the Australian National University and the Climate Council.

As time went on, 2005 was hotter than 1998, 2010 hotter than 2005, and 2014, 15, 16, 17 and 18, hotter than all of them.

1998 now clings on to 10th place in the global temperature books, with the hottest nine all since 2005.

But that pesky trendline was right all along. ( Supplied: NASA )

Unless it's a seriously anomalous year, 2019 looks almost certain to knock 1998 out of the top 10.

"The climate has warmed very strongly over the past 30 years," Professor Steffen and Dr Dean said.

"These short fluctuations do not show the long-term trend in climate."

It's a natural cycle, man

Carbon dioxide levels for as far back as our ice cores can show have never been as high as they are today. ( Supplied: NASA )

Earth's climate is constantly changing.

Typically that's due to things like Milankovitch cycles, variations in solar activity, as well as changes in greenhouse gas quantities in the atmosphere, and it mostly happens on timescales of tens to hundreds of thousands of years.

The slow nature of these cycles mean plant communities are able to migrate north and south, and animals — humans recently included — can go with them.

But there have also been some really fast warming events.

The biggest one we know of was the end-Permian extinction, also known as the "Great Dying", about 252 million years ago.

It's called the Great Dying because about 90 per cent of species died.

Researchers believe it was caused by CO2 from massive volcanic eruptions in Siberia, wildfires, plus high emissions of methane.

The CO2 emissions that kickstarted the end happened over a period of between 2,100 and 18,800 years.

The average temperature on Earth ramped up by up to 10C.

We're hoping that's a much more radical change than we're going to face — the IPCC's worst-case scenario is 4.8C by the end of the century.

But at least in part, the ferocity of the end-Permian extinction was likely due to the rate Earth warmed at.

Then, like now, species didn't have a chance to adapt to rapid changes in climate.

Unlike the end-Permian, volcanic activity on Earth right now is pretty low by geological standards, yet CO2 is rising faster and higher than it has been for as far back as our ice cores can show — about 800,000 years.

And we know that's because we're burning fossil fuels, according to Professor Howden.

"[CO2] concentrations were 280ppm in the pre-industrial era, and they're now at 415ppm," he said.

"It's definitely due to humans because the isotopic signature of carbon dioxide in the atmosphere has changed and it reflects the fact that additional carbon dioxide is being added from fossil fuels."

"[Carbon dioxide from] fossil fuels has a different isotopic signature from carbon dioxide from say, decomposing trees or grass."

Plants grow better with more CO2 anyway

Setting aside the fact that the biggest limitation to tree growth isn't CO2, but humans cutting them down, on face value there is some truth to this argument.

Carbon dioxide makes plant food. And like people, when plants eat more food they get bigger.

But also like humans, plants don't need just one type of food, they need variety in order to get their fill of nutrients.

For plants that's things like phosphorus, nitrogen, potassium, calcium, magnesium and sulfur.

In Australia, and other continents like South America, phosphorus is the big limiting factor.

David Ellsworth, an environmental scientist from Western Sydney University, exposed areas of mature eucalypt forest to high levels of CO2 for three years.

While there was more photosynthesis in the leaves, the trees didn't grow any more than the control groups that were exposed to ambient air, he told the ABC earlier this year.

"Growth is limited by nutrients in the soil," Professor Ellsworth said.

"What we've found is that low phosphorus levels really prevent the kind of growth that predictions would tell you to expect."

But trees that were given extra phosphorus and no extra CO2 increased their growth rate by about 35 per cent.

Increased CO2 will only make a difference to tree growth if we distribute fertiliser with it.

It was warmer during the Medieval Warm Period

This one goes hand-in-hand with the natural-cycles argument and is one of the more logically challenged zombies.

Palaeoclimate data [mostly proxy data from coral and tree records] shows warmer than average conditions, mostly in Europe and North America for at least some parts of AD 800-1200 — known as the Medieval Warm Period.

But analysis of the data published this week in the journal Nature shows two things.

Firstly, that Earth wasn't uniformly warmed during that period. Peak warm periods occurred at different times in Europe than in North America.

According to the researchers, framing these localised temperature peaks as representing globally warmer conditions is misleading.

"The a-priori belief about a given palaeoclimate time series should be that it represents local information," they write.

Second, the research confirms that temperatures during the Medieval Warm Period weren't as warm as today.

Instead, the researchers reiterated that the hottest period between AD 1 and AD 2000 was in the tail end of the 20th century, and that unlike between AD 800-1200, that warming has been uniform across 98 per cent of the globe at the same time.

This also indicates that current CO2 climate forcing is stronger than the forcing between AD 800-1200.

But the world's been colder before when CO2 levels were higher

See above.

CO2 isn't the only driver of climate.

Actually CO2 has been at higher levels than today, when temperatures were much cooler.

That's when things like low solar activity or Milankovitch cycles were the dominant climate forcings, or factors.

There are a multitude of things that can affect the climate, and CO2 is just one of them.

When all things are equal, and you take one factor and ramp it up or down, it's going to make a change.

We can observe this from the Pliocene epoch, a mere 5.3 to 2.6 million years ago.

Background climate factors were similar to today, and like today, CO2 was around 400 parts per million.

At the time, temperatures were about 3C to 4C warmer than today, sea levels were about 20 metres higher, and there were trees in Antarctica.

Climate science is a conspiracy of the 'elites'

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This one has many guises but basically says that there's a group of governments and global elites that stand to benefit from hoaxing the world into believing the climate is changing.

It's not that we should inherently trust governments, but instead that we should have a healthy scepticism of their competence.

To pull off this feat, so the conspiracy goes, they've conscripted groups including NASA, the Australian Bureau of Meteorology, the UK Met Office, the CIA, thousands of climate scientists and universities wordwide, among a raft of others.

A well-known internet meme summarises it thusly:

"The plot: 97 per cent of the world's scientists contrive an environmental crisis, only to be exposed by a plucky band of billionaires and oil companies."

If you meet this zombie, the best advice is to run.

The reality is that not acting on climate change will punish the poor. The IPCC predicts millions will be displaced if warming hits 2 degrees.

Many of those will be in low-lying areas of developing nations including Bangladesh, and Australia's island neighbours in the South Pacific.

Increases in extreme weather events also disproportionately affect areas with poor housing and infrastructure, and where subsistence farmers can be quickly undone by drought.

CO2 has historically followed, not caused warming

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This zombie mistakenly tries to discredit climate science by looking for holes in Al Gore's movie an Inconvenient Truth. But it's just a movie, zombie.

It is true that CO2 has risen after the temperature has started going up, during some periods of warming throughout history.

But the conclusion that therefore CO2 doesn't cause warming, isn't sound.

As discussed above, there are many forcings that cause climate change.

And there are things called positive feedback loops.

So for instance, deglaciation that began around 20,000 years ago was initially caused by changes in Earth's orbit of the Sun.

This initial temperature increase warmed the oceans, and warmer oceans, as research shows, release more CO2.

That CO2 then increased in the atmosphere, and in turn created even more warming, that in turn released more CO2 from the oceans — a positive feedback, according to Professor Steffen and Dr Dean.

"These positive carbon cycle feedbacks initially follow temperature rises, but then amplify them by causing further warming."

And CO2 isn't the only greenhouse gas that is involved.

"Warmer temperatures can also melt permafrost [frozen subsoil] in the Arctic, causing the release of methane, a potent greenhouse gas," they said.

Climate scientists are only in it for the money

Back in the mid-1990s, a now controversial Hollywood actor paraphrased Baudelaire: "The greatest trick the devil ever pulled was convincing the world he didn't exist."

If you were born around the time that movie was released, and you'd decided to study an environmental science or related degree at university, you'd be finishing your undergraduate degree about now and facing a choice.

On the one hand, you could take a graduate job in the mining sector where you could earn anything from $51,000 to $120,000+, for fly-in-fly-out and remote work in your first year out of university.

On the other, you could stay and complete an honours year which pays nothing.

If masochism is your thing, having completed that honours year, you could then take on a three-year-minimum PhD, which also pays the princely sum of nothing.

With your PhD complete and having spent a minimum four years earning next to nothing, you could then apply for post-doctorate research positions.

But they still probably won't pay as well as that graduate job with the mines. Which by the way is now paying much more because you've been there going on five years and you're no longer on a graduate's salary.

Apart from the fact that climate scientists don't earn any more than other researchers, the idea that you go into research for the money is the greatest trick the anti-climate-science lobby ever pulled.

As Professor Steffen and Dr Dean pointed out, science "isn't a particularly lucrative profession".

"According to the ABS, people employed in full-time jobs related to professional, scientific and technical services earned an average of $1,872 per week in the year to November 2018," they said.

"This is more than some industries, but less than some other industries, such as mining, which paid on average $2,696 per week to full-time employees over the same period."

The fossil fuel industry is worth nearly $5 trillion, according to a Bloomberg New Energy Finance report from 2014.

Like with most things, when it comes to climate science, the devil is in the details.