From temperature change to sea level rise, climate science is full of contradictory figures.

When the ice melts, the sea level will rise. But how big is the increase and how fast is it?

If we don't take active action now, global warming will be 3 degrees Celsius, or even 5 degrees Celsius ... Will the sea level rise by 0.3m or 3m by 2 100?

Almost every article you will read about climate change is full of numbers. Starting from 1.5 degrees Celsius, we are told that these figures represent the maximum temperature rise we can allow and avoid the worst effects of global warming.

But this is not the case, this is just the beginning of chaos. It seems that no two figures in climate change research are consistent. Even climatologists are often confused by the data obtained by other researchers.

People who deny climate change seize uncertainty as evidence of potential scientific errors. Actually, it's not. It's complicated, just like science in the chaotic real world. The biggest uncertainty so far is what we are, that is, what we will do in the next century. The impact of uncertainty is two-way: we may underestimate the speed and impact of global warming.

So, which figures can be determined and which figures cannot be determined?

How much has the earth warmed?

As part of the Paris Climate Agreement of February 20 15, 15, almost every country in the world agreed to limit the global average temperature increase to 1.5℃ higher than the pre-industrial level.

To find out what this means, we must first know where we are now. The uncertainty starts here. Before crossing the threshold, we may still have a temperature as high as 0.6℃ or less than 0.3℃.

How can we not know how much the world is getting warmer? Then, the best way to measure global warming is to look at global warming from the perspective of land, sea and atmosphere. But our measurement focuses on the thin layer where we live: the global average surface temperature usually refers to the heat of air 2 meters above the ground.

We know how this temperature changes from thousands of weather stations on land and ships and buoys at sea. Every reading will be checked to see if the temperature is lower than or higher than the long-term average temperature at the same time and place in a year. Then combine them to calculate the change of global average surface temperature relative to long-term average or baseline.

It's not easy. For example, although weather stations on land measure air 2 meters above the ground, the ocean usually observes the sea surface temperature. And because you can't build a fixed weather station in an ocean full of moving glaciers, we have almost no measurement data of the North Pole. The temperature records kept by the Met Office only exclude the North Pole. NASA's GISTEMP records estimate the temperature of the Arctic based on surrounding weather stations. Due to the rapid warming of the Arctic, NASA data show that the degree of global warming is nearly 0. 1 degree Celsius higher than that of the British Meteorological Office.

Further confusion around the baseline issue. The earliest record of global temperature can be traced back to 1850, but the industrial age began a century ago. Computer models show that before 1850, the world temperature rose by 0.2 degrees Celsius.

Nevertheless, the average temperature from 1850 to 1900 is considered as a semi-official "pre-industrial level", because this is the earliest period when we have direct measurement. Ed Hawkins of the University of Reading in the UK participated in many related studies. He said, "I don't think society has much interest in changing this situation."

If we use the records of the Meteorological Bureau and take 1850 to 1900 as the baseline, then the global warming temperature so far is about 0.9℃. Using NASA and the earlier pre-industrial baseline, it is already 1.2 degrees Celsius.

With the global warming trend far exceeding 1.5℃, this difference becomes very small. But if we really want to limit the temperature rise to 1.5℃, it is really important. Hawkins said: "Take 1.5 as an example, the difference of 0. 1 is a very large number."

What is the safe limit of temperature rise?

Not 1.5 c The reason why 1.5 C is used is not because it is the correct number, but because it is convenient. A report in 1990 concluded that it was safer to limit global warming to 1℃ than the upper limit of 2℃, but in 1996, because 1℃ was out of reach, the EU Council of Ministers set 2℃ as the target. This led to its adoption by the United Nations in 20 10.

20 15 when the Paris climate agreement was negotiated, the island countries facing floods needed a lower temperature target. Because the world temperature had already risen by about 1℃, 1.5℃ was chosen. But as the report of the Intergovernmental Panel on Climate Change (IPCC) on this goal in June 5438+ 10 shows, this is not a security restriction.

More and more evidence shows that climate warming is contributing to record-breaking extreme phenomena. We witnessed devastating storms, incredible heat waves and raging wildfires. What is safe is now considered to be only a matter of degree. As the world gets hotter and hotter, most adverse factors of global warming, from coral bleaching to more serious floods, will become more and more serious.

There are also some potential turning points, such as the closure of the Atlantic Ocean current that warms northern Europe. However, because we are not sure at what temperature these substances will work, it does not help to establish a "safe" limit.

The danger of any particular degree of warming depends on us to some extent. If we stop building houses on the coast doomed to disappear in the waves and start adjusting our homes to cope with more extreme weather, we will save many lives.

When will we exceed the limit of 1.5 degrees Celsius?

According to the current trend, the first year when 1.5℃ is higher than the average value of 1850 to 1900 is likely to appear in 2020. But because the climate is the average weather for many years, it is too early to think that it exceeds this limit. During El Nino, this threshold may be crossed, during which warm seawater crosses the Pacific Ocean, temporarily raising the global surface temperature. When all this is over, the temperature will drop a little.

A reasonable definition is that when the average and long-term temperature rise exceeds 1.5℃, we exceed this limit. According to the current trajectory, this is likely to happen around 2040, and many scientists and politicians have adopted slightly different definitions of reaching 1.5℃. Almost all scenarios about the target of 1.5℃ in the report of the Intergovernmental Panel on Climate Change involve that after the threshold of 1.5℃ is exceeded in the middle of this century, the temperature will rise below this threshold in 2 100.

If you accumulate a large amount of temperature debt, the only way to pay off the debt is to reduce the carbon dioxide content in the atmosphere by removing a large amount of carbon dioxide, that is, to restore the temperature to its original level after the temperature is too high. At present, there are many ways to capture carbon in a small scale, but none of them can reverse the trend of large-scale combustion of fossil fuels for decades.

Even if the temperature can be lowered, if it exceeds 1.5℃, the impact will be more serious, because the temperature will rise faster in the next few decades. This may trigger some critical points that cannot be quickly reversed, such as the disappearance of the Amazon rainforest.

How much warming will carbon dioxide cause?

This is probably the most intractable problem in all climate science. Carbon dioxide can easily directly warm the earth by absorbing more solar heat.

But it also triggered all kinds of feedback that affected the global temperature. For example, the intensification of warming means more water vapor, which is a powerful greenhouse gas. The behavior of the cloud will also change immediately.

Other feedbacks take thousands of years. For example, as the huge reflective ice sheet melts, climate warming will be amplified and replaced by land and water that absorb most of the sunlight. What is even more puzzling is that we are discharging various other pollutants into the atmosphere, some of which have cooling effect. This not only makes it difficult for us to determine the degree of carbon dioxide warming, but also makes it difficult to calculate what we need to do to limit warming, because it also depends on the degree of change of these pollutants.

A common measure of climate sensitivity is how much warming will occur in the decades or centuries after the carbon dioxide level doubles. The results show that the most likely value is 3 C, but the possible value ranges from below 2 C to above 5 C. If the emissions continue to increase, in less than 50 years, the level of carbon dioxide will be twice the pre-industrial level.

Although we are increasingly convinced that low values within a reasonable range can be excluded, the long tail of high values cannot be excluded. Earlier this year, some climate scientists warned that we may have greatly underestimated this risk, and if the earth does warm at least 2 degrees Celsius, it may not be able to stop it from warming a few more degrees.

How much carbon dioxide can we emit?

Although we can't determine the exact value of climate sensitivity to carbon dioxide and other greenhouse gases, it is obvious that what matters is their content in the atmosphere. To simplify the problem, climate scientists have begun to talk about the carbon budget: how much carbon dioxide we can emit.

Because carbon dioxide has stayed in the atmosphere for many centuries, there is a direct relationship between the total and cumulative emissions of carbon dioxide and its content in the atmosphere. So what is the budget of 1.5 C?

Glenn Peters of the Norwegian Center for International Climate Research says that most carbon budgets are "over budget". These show how much carbon dioxide we can emit before the temperature rises to1.5 C.

A report of the Intergovernmental Panel on Climate Change (IPCC)20 13 estimates that the possibility of limiting the warming to 1.5℃ is 66%, and the carbon budget of 2011800 million tons of carbon dioxide. In the latest report, the figure is 420 billion tons.

The higher budget is partly the result of some really good news. The early calculation relied on the estimated emissions in the past century and used a computer model with a slightly higher temperature rise. More accurate figures and better models have led to an increase in carbon dioxide emissions.

However, the latest report of the Intergovernmental Panel on Climate Change acknowledges that the budget may be very different for the reasons we have already considered. For example, using different temperature records can make the budget as low as 258 billion tons or as high as 570 billion tons.

Even these figures mask great uncertainty. According to climate sensitivity and historical baseline, the budget may be 650 billion tons or higher. In addition, the report also said that if wetlands release more methane and melting permafrost releases more carbon, the budget will be 654.38 billion tons lower than expected.

If it is bungee jumping, this budget is equivalent to the length of the rope. You need to limit the safe length of the rope to avoid being hit to the ground.

How high will the sea level rise?

During the warm period of the ice age about 1.20 million years ago, the temperature was about 1.850 ~ 1.900 years higher, and the sea level was 6 ~ 9 meters higher. In other words, even if we limit the warming to 1.5℃, most of the ice in Greenland and West Antarctica may still disappear, which will be enough to raise the sea level by 5 meters or more.

If we are actually moving towards the possibility of a temperature rise of 3 or 4 degrees Celsius, this will lead to a sea level rise of more than 20 meters. Because the temperature of the earth is rising much faster than in any recent warm period, the study of the past temperature of the earth is not a good guide for our future.

The general view is that it will take many centuries or thousands of years. The prediction of the Intergovernmental Panel on Climate Change is that the sea level will rise by 0.3-0.8m in the world where the temperature rises 1.5℃, and will rise by 0.5-1 m by the end of this century if emissions are not controlled. If the weather stays warm, the sea level will rise even more in the 22 nd century and beyond.

Some scientists think these predictions are conservative. The ice in Antarctica is melting much faster than expected. A study based on the 20 16 ice sheet computer model shows that the sea level may rise by 3 meters by 2 100.

How long will it take us to turn things around?

"Scientists say that we still have 12 years to save the world." This seems to be the information that many people got from the latest report of the Intergovernmental Panel on Climate Change (IPCC)-but the report doesn't say so.

In fact, according to the current emissions, we will exceed the "most likely" surplus carbon budget in the report in about 12 years. However, we can see that the carbon budget is in the middle of a wide range, and the goal of1.5 C is itself an arbitrary goal.

People who deny climate change will point out that they have been told many times before that the time to save the earth is only "XX" years. Therefore, focusing on arbitrary deadlines may not be the best way to summarize science.

Pierre Foster of the University of Leeds in England said, "I personally don't like this 12 year. In fact, we must immediately take greater action than ever before. "

In the quagmire of confusing and contradictory figures, two things are still clearly visible. First of all, we must reduce the global net emissions to zero. The sooner we do this, the better our life will be. Secondly, how bad things get depends in part on how much preparation we have made. We need to seriously adapt to life on a warmer planet.