Evidence for Climate Change: Is the Earth Really Heating Up?
Hello everyone! Welcome to your study notes on climate change. It's a huge topic you hear about in the news all the time. But how do we actually know the climate is changing? Is it just a normal phase for Earth, or is something different happening now?
In this chapter, we're going to be detectives! We'll look at the clues and evidence scientists use to understand what's happening to our planet. We'll examine:
- Temperature Records: The Earth's 'fever chart'.
- Ice Cores: Frozen time capsules from the past.
- Sea-Level Rise: Why the oceans are getting higher.
- Other Clues: Like melting ice and wild weather.
Understanding this evidence is super important because it helps us answer the big question from the syllabus: Is global warming a long-term fluctuation (a natural cycle) or an irreversible trend (a permanent change, likely caused by us)? Let's get started!
1. Temperature Records: Taking the Earth's Temperature
The most direct way to see if the world is getting warmer is to, well, measure the temperature! Scientists have been doing this for a long time.
What are Temperature Records?
These are actual temperature measurements collected from thousands of weather stations, ships, and satellites all over the world. When scientists put all this data together, they can calculate the mean global temperature (the average temperature for the entire planet for a year).
What do the records show?
The evidence is very clear:
- There has been a sharp increase in the mean global temperature, especially in the last few decades (since around 1980).
- Most of the warmest years ever recorded have all happened in the 21st century.
- This warming trend is happening almost everywhere on the globe.
A Local Example: Hong Kong
We can even see this in our own city! Data from the Hong Kong Observatory shows that Hong Kong is getting warmer. The number of 'Very Hot Days' has been increasing over the past decades.
More than just averages: Heat Waves
A direct result of rising average temperatures is an increase in heat waves. These are long periods of unusually hot weather, which can be very dangerous. We are experiencing these more often and with greater intensity.
Think of it like this: If your average grade in school goes up from 70% to 75%, it doesn't just mean all your grades were 75%. It means you probably got more grades in the 80s and 90s than before. In the same way, a small rise in the global average temperature leads to a big increase in extreme heat events.
Key Takeaway
Direct temperature measurements from all over the world show a clear and rapid warming trend in recent decades. This is the most direct evidence of global warming.
2. Ice Cores: A Frozen History Book
How do we know if the current warming is unusual? We need to look into the past! But how can we know what the temperature was 100,000 years ago? The answer is frozen in places like Antarctica and Greenland.
What are Ice Cores?
Scientists drill deep into ancient ice sheets and pull out long cylinders of ice called ice cores. These cores can be several kilometres long!
The ice sheet is made of layers of snow that fell year after year and got compressed into ice. The bottom layers are very old, and the top layers are new. It's like a history book, where each layer is a different page.
What can we learn from them?
These frozen layers tell us two very important things about the past climate:
- Past Temperatures: By studying the water molecules in the ice, scientists can figure out the temperature when the snow fell. (Don't worry about the complex chemistry, just know that the ice itself is a natural thermometer!).
- Past Atmosphere: Tiny air bubbles are trapped in the ice. These bubbles are perfect little samples of the ancient atmosphere! Scientists can measure the amount of greenhouse gases, like carbon dioxide (CO2), that were in the air thousands of years ago.
The "Smoking Gun" Evidence
When scientists plot the temperature data and the CO2 data from ice cores on a graph, they find something amazing:
Temperature and CO2 levels go up and down together almost perfectly through history.
And the most important finding? Today's CO2 levels, measured from the real atmosphere, are much, much higher than at any point in the last 800,000 years of the ice core record. This shows that the recent spike is not part of the normal natural cycle.
Key Takeaway
Ice cores provide long-term evidence showing a strong link between CO2 and temperature. They prove that current CO2 levels are unnaturally high compared to the past, suggesting the current warming is not just a normal fluctuation.
3. Sea-Level Rise: The Overfilling Bathtub
Another major piece of evidence is that the average level of our oceans is rising. Satellite measurements confirm this is happening and that the speed of the rise is increasing.
Why is the sea level rising?
There are two main reasons, both caused by global warming. Think of the ocean as a bathtub:
1. Thermal Expansion: This is the biggest cause. When water gets warmer, it expands and takes up more space. So, as the ocean absorbs heat from the atmosphere, it literally expands. This is like the water level in the tub rising simply because you turned on the hot tap.
2. Melting of Land-Based Ice: This is the second cause. When huge glaciers (in mountains) and ice sheets (in Greenland and Antarctica) melt, the meltwater runs into the ocean, adding more water. This is like adding more water to the tub from a bucket.
A Common Mistake to Avoid!
Does melting sea ice in the Arctic raise sea levels? No, not really.
Arctic sea ice is already floating in the ocean. Think of an ice cube in a glass of water. When it melts, the water level doesn't rise because the ice was already displacing its own weight in water. It's the ice melting on LAND that adds new water to the ocean and causes sea levels to rise.
Impact: Coastal Flooding
Even a small rise in sea level can have big effects. It leads to more frequent and more severe coastal flooding, especially during storms and high tides. This is a huge threat to coastal cities around the world, including parts of the Pearl River Delta.
Key Takeaway
Sea levels are rising because of thermal expansion of ocean water and melting land-based ice (glaciers and ice sheets). This is strong evidence of a warming planet and has serious consequences like coastal flooding.
4. Other Clear Clues: What We Can See Happening
Melting of Glaciers
Around the world, from the Alps to the Himalayas, glaciers are shrinking at an alarming rate. We don't even need complex instruments for this – we can see the difference in before-and-after photographs taken decades apart. This widespread retreat of glaciers is a very visible sign of a warming world.
More Frequent Extreme Weather Conditions
Is the weather getting weirder? The evidence suggests yes. While climate change doesn't "cause" a single storm, it makes many types of extreme weather more likely and more intense.
- A warmer atmosphere can hold more moisture, which leads to heavier rainfall and floods.
- Warmer ocean surface temperatures provide more fuel for storms, potentially leading to more intense typhoons.
- Changes in weather patterns can also lead to more severe droughts in other regions.
Important: It's wrong to say "Typhoon Mangkhut was caused by climate change." But it's correct to say "Climate change is making powerful typhoons like Mangkhut more likely to occur."
Key Takeaway
The widespread melting of glaciers and an observed increase in the frequency and intensity of extreme weather events (like heat waves, heavy rain, and intense storms) are further signs that our planet's climate is changing.
The Big Picture: Fluctuation or Irreversible Trend?
So, let's return to our main question. Is this all just a natural cycle?
The 'Natural Fluctuation' Argument
Some people argue that the Earth's climate has always changed. We've had ice ages and warm periods long before humans existed. This is true. The ice cores show these natural cycles. So, perhaps the current warming is just another one of these.
The 'Human-Induced Trend' Argument (Supported by the Evidence)
However, the vast majority of scientists conclude that the current situation is different. Here's why the evidence points to a human-caused trend:
- The Speed of Change: The current warming is happening much, much faster than most of the natural changes we see in the geological record.
- The Unprecedented CO2 Levels: As the ice cores show, CO2 levels today are far higher than they have been for nearly a million years. This spike began right at the time of the Industrial Revolution when humans started burning massive amounts of fossil fuels.
- The Clear Correlation: The sharp rise in global temperature directly matches the sharp rise in human-emitted greenhouse gases.
While natural fluctuations are always happening, the evidence strongly suggests that human activities have pushed the climate system into a new, rapidly warming state, creating an irreversible trend on top of any natural cycles.