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A number of business and energy groups are calling for all sides of politics to seize the opportunity and come up with a climate policy which is "enduring and credible."

Over the past few days there has been a lot of talk about the Paris climate agreement, from which the United States is planning to withdraw. Although this is a setback, there is still near-complete consensus from the world’s governments that a strong effort to tackle climate change is needed.

The Paris Agreement aims to limit global warming relative to a pre-industrial baseline. Its precise commitment is:

Holding the increase in the global average temperature to well below 2℃ above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5℃ above pre-industrial levels, recognising that this would significantly reduce the risks and impacts of climate change.

But this begs the question: what are “pre-industrial levels”?

Clearly, if we’re aiming to limit global warming to 1.5℃ or 2℃ above a certain point, we need a common understanding of what we’re working from. But the Paris Agreement doesn’t provide a definition.

This becomes key as governments expect climate scientists to coherently compare different plans to reach their Paris targets. It’s crucial to be clear on what researchers mean when we say “pre-industrial”, and what assumptions our projections are based on.

Of course, as the chart below shows, no matter which baseline we use it’s clear there’s been a drastic rise in global temperature over the last century.

Global temperatures are on the rise and are about 1℃ above late 19th century levels. Author provided Defining a pre-industrial baseline

The Industrial Revolution began in the late 1700s in Britain, and spread around the world. But this only marked the beginning of a gradual rise in our greenhouse gas emissions. Various studies have found climate change signals appearing on a global scale as early as the 1830s, or as recently as the 1930s.

Besides the evolving and increasing human influence on the climate, we also know that plenty of other natural factors can affect Earth’s temperature. This natural variability in the climate makes it harder to determine a single precise pre-industrial baseline.

Scientists separate these natural influences on the climate into two groups: internal and external forcings.

Internal forcings transfer heat between different parts of Earth’s climate system. The El Niño-Southern Oscillation, for example, moves heat between the atmosphere and the ocean, causing year-to-year variations in global average surface temperatures of about 0.2℃. Similar variations also happen on decadal timescales, which are associated with slower energy transfers and longer variations in Earth’s temperature.

External forcings come from outside Earth’s climate system to influence global temperature. One example of an external forcing is volcanic eruptions, which send particles into the upper atmosphere. This prevents energy from the Sun reaching Earth’s surface, and leads to a temporary cooling.

Another external influence on Earth’s climate is the variability in the amount of energy the Sun emits.

The Sun’s total energy output varies on multiple cycles and is related to the number of sunspots, with slightly higher temperatures when there are more sunspots, and vice versa.

Earth has experienced extended periods of cooling due to more frequent explosive volcanic eruptions and periods of few sunspots – such as during the “Little Ice Age” which lasted roughly from 1300 to the 1800s.

There is high variability in the solar and volcanic influences on the climate (top row) while greenhouse gas influences rise over time (bottom row). A suggested 1720-1800 baseline is highlighted in grey. Adapted from Hawkins et al. (2017).

All of these factors mean that Earth’s climate can vary quite substantially even without human interference.

It also means that if we choose a pre-industrial baseline when there was low solar activity, like the late 1600s, or in a period of high volcanic activity, like the 1810s or the 1880s, then we would have a lower reference point and we would pass through 1.5℃ or 2℃ sooner.

A challenge not only for scientists

At the moment there is a drive among the climate science community to better understand the impacts of 1.5℃ of global warming. The Intergovernmental Panel on Climate Change will deliver a special report on 1.5℃ next year.

But scientists are defining “pre-industrial” or “natural” climate in different ways. Some work from the beginning of global temperature records in the late 19th century, while others use climate model simulations that exclude human influences over a more recent period. One recent study suggested that the best baseline might be 1720-1800.

These different definitions make it harder to synthesise the results from individual studies, which is vital to informing decision-making.

This will have to be a consideration in the writing of the IPCC’s report, as policymakers will need to easily compare impacts at different levels of global warming.

There is no definitive way to determine the best “pre-industrial” reference point. An alternative might be to avoid the pre-industrial baseline altogether, and instead set targets from more recent periods, when we have a better grasp of what the global climate looked like.

You can read more about defining a pre-industrial climate here and here.

Andrew King receives funding from the ARC Centre of Excellence for Climate System Science.

Ben Henley receives funding from an ARC Linkage Project and is an associate investigator with the ARC Centre of Excellence for Climate System Science.

Ed Hawkins receives funding from the UK's Natural Environment Research Council and National Centre for Atmospheric Science.

Exclusive: On the eve of the long-awaited Finkel review, analysis shows Australia’s emissions rose sharply in the first quarter of 2017

Australia’s carbon emissions jumped at the start of 2017, the first time they have risen in the first few months of a year for more than a decade, according to projections produced exclusively for the Guardian.

Emissions in the first three months of the year normally drop compared with the previous quarter, driven by seasonal factors and holidays. But in something not seen in since 2005, emissions rose in the first quarter of 2017 compared with the last quarter of 2016 by 1.54m tonnes of CO2, according to the study by consultants NDEVR Environmental. The rise was driven by increases in emissions from electricity generation.

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In a change of tone, Utah governor Gary Herbert has backpedaled and asked the federal government not to allow oil and gas drilling around the famous park

When Utah governor Gary Herbert changed his mind last week and decided oil and gas companies should not be allowed to drill near Zion national park, it seemed like a remarkable change of tone.

The Republican has been a staunch advocate for rolling back public land protections and had earlier endorsed the idea of drilling near the 229 sq mile park. In February, he signed a resolution urging Donald Trump to rescind national monument status for the 1.3m acres known as Bears Ears in south-eastern Utah. Doing so would allow expansion of current leases for oil and gas development and grazing.

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In just 31 hours, Guardian US exceeded its target to support a new series on the threat to America’s public lands. We’d like to thank you for your generosity

• In Montana, land transfer threatens the American rancher’s way of life
  

We would like to extend a huge thank you to the more than 1,000 Guardian readers who made contributions to support This Land is Your Land, our series on the threat to America’s public lands. We launched our fundraising campaign on Monday morning, and hit our $50,000 goal at 1pm Tuesday, just 31 hours after the launch. When this article was published we had exceeded our goal by 20%, with $60,166 pledged. More than 1,000 readers have contributed.

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HUMAN EVOLUTION: The discovery of fossil remains of Stone Age humans in Morocco pushes back the date of the origin of Homo sapiens by 100,000 years.

Nuclear, wind and solar power in UK generate more electricity than gas and coal combined for first time ever

The windy weather across Europe in the past 24 hours may have been a curse for summer picnics, but it has set records for renewable power.

Related: 'Spectacular' drop in renewable energy costs leads to record global boost

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Fossils of modern humans uncovered in north Africa are at least 300,000 years old.

A peacock which found its way into a Californian liquor store causes hundreds of dollars of damage before an animal control officer and the store manager are able to capture it

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The human-elephant conflict plays a huge role in the rapid decline elephant numbers. A panel of experts share ideas on how to mitigate this problem

We need conservation (not just fighting the illegal wildlife trade which has captured the limelight more recently) to be far higher up the political agenda. For example, we in the UK could be much more effectively linking overseas aid (budget of £12bn this year) to poverty relief, sustainable development and environmental protection, ecosystem services and conservation priorities. Will Travers, president, Born Free Foundation

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China, US and Europe accounted for more than 90% of electric vehicle sales last year with decreasing costs driving demand

The number of electric cars in the world accelerated past the 2m barrier last year, as prices fell and manufacturers launched new models.

The number of battery-powered vehicles numbered just hundreds globally in 2005 and passed the 1m milestone in 2015, but sales jumped 60% in 2016.

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• Princeton-Rutgers study finds sharp increase in risk of frequent deluges
• ‘Many cities are behind the eight-ball in terms of preparing for flooding’

US coastal areas are set to be deluged by far more frequent and severe flooding events if greenhouse gas emissions aren’t slashed, with rare floods becoming the norm for places such as New York City, Seattle and San Diego, new research has found.

Related: Climate change progress at Trump's EPA is grinding to a halt, workers reveal

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David Simon asks for a meat tax to cover the hidden costs relating to animal suffering, damage to the environment and healthcare.

Parisian Clotilde Nonnez has lived in the capital for 30 years and has seen her health deteriorate.

What's the difference between a huntsman and a wolf-spider? A redback and an orb-weaver?

The permanent reoccupation of the UK's Halley station depends on how an ice crack develops.

The Halley VI Research Station was moved more than 20km across the ice sheet.

A growth in demand for elephant parts to be used in traditional medicine in Asia means the number of elephants being killed in Myanmar is rising

Case files and laminated photos of poachers spill out of captain Than Naing’s folder. As the chief of police in Okekan township, one of Myanmar’s recent poaching hotspots, he is trying to track down the men who have killed at least three elephants in the area over the past year. So far, he has arrested 11 people suspected of having assisted the poachers. Meanwhile the poachers themselves remain at large.

“These are the two men who we believe killed one of the elephants,” he says, pointing to two photos. “They are still on the run.”

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Mizen Head to Malin Head is a 510-mile trip that takes in the breathtaking Maumturk Mountains, the empty beaches of Cork and Kerry, and the wilderness of the Burren

Send anyone to the west coast of Ireland and they will fall in love with it at some point. For me it was at a junction in Maum, County Galway, where I had stopped to take a photo of a signpost but ended up having my breath taken away by the Maumturk Mountains in all their pastoral, sunlit glory.

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Guardian experts give their view on the main parties’ public service manifesto pledges. Here, our environment editor looks at energy, pollution and recycling

• Denis Campbell on health
• David Brindle on social care
• Patrick Butler on social security
• Dawn Foster on housing
• Anna Bawden on local government
• Frances Ryan on disability
• Alan Travis on criminal justice and immigration
• Jane Dudman on the civil service
• Scotland, Wales and Northern Ireland

New support for fracking to extract shale and coal seam gas is the most striking pledge from the Conservatives, with the easing of planning rules, a new dedicated regulator and more of any future tax revenues going directly to communities hosting shale gas sites. Wind power remains ruled out in England, but offshore wind farms are supported. The energy efficiency of all fuel-poor homes would be upgraded to meet energy performance certificate (EPC) band C criteria by 2030. There is no environment section in the manifesto and the UK’s air pollution crisis gets a single sentence: “We will take action against poor air quality in urban areas.” A free vote on repealing the ban on fox hunting with dogs is promised.

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