# The Climate Change Skeptic’s Argument: Natural Solar Cycles or Human Activity?

## Explore

Over the span of 11 years, the Sun’s activity waxes and wanes as its magnetic field lines associated with sunspots periodically break through to the surface. Read, “The Role of Sunspots and Solar Winds in Climate Change” to read about what scientists and skeptics think about sunspots and their role in climate change.

What does the data on sunspots and Total Solar Irradiance look like and what can it tell us? In this section, you will analyze a historical record of sunspot activity and solar irradiance for its potential role in contemporary climate change.

### Data Activity: Sunspots and Total Solar Irradiance

These activities guide you through an analysis of sunspot data, compiled from both instrumental data and historical records. Remember, data provides the backbone support of a STEM lesson and as you complete the following activities, you should think about how you may implement these data activities with your students.

Review the “Sunspots Since 1749” graph of sunspot activity since 1749. The data was collected by the Royal Greenwich Observatory. An 11-year moving average trend line describes the data variability fairly well and the 11-year sun spot cycle has been understood for some time. Calculating the linear trend mathematically finds the straight line that summarizes the slope of the data. The equation for the line (y=0.0845x - 106.) is plotted at the top of the chart.

1. Do you notice any periods of abnormally low or high sunspot numbers in the graph?

The period during the first half of the 1800’s shows a markedly lower frequency of sunspots and there is another minimum centered around 1900. There is a slight increase (positive slope) for the trend line, indicating gradually increasing numbers of sunspots over the period of record.

Next, analyze “Sunspots Since 1749 and Mean Annual Temperature”. This plot overlays global mean annual temperature (°C, shown in red) with the historical record for sunspot activity (shown in blue). The 50-year moving average is shown in black and the 11-year moving average is shown in green.

1. Do you see any correlation between the recent temperature increases and increased solar activity as measured by sunspot activity? Look at both the overall trend as well as the details.

From these data, it is not possible to determine how much of the warming trend during times of increased solar activity is the result of increased solar variability. However, the temperature has been steadily increasing for the past two centuries, while solar activity cycles from high to low to high many times during the same period. Because of the radiation budget, we know that there is not a cumulative impact on climate from solar cycles, which suggests that a factor besides solar activity must be responsible for the rise in temperature.

When looking at the graph of “Total Solar Irradiance Reconstruction (W/m2)” since 1610, notice that the red line represents a 100-year moving average and the black line is a linear trend line of the data. This plot of data extends the record back in time to include the Little Ice Age.

1. Where do you think the Little Ice Age is indicated on the graph?

The Little Ice Age occurred during the minimum in solar activity during the latter portion of the 1600’s, and is clearly visible on the graph.

2. What does the rate and trajectory of Total Solar Irradiance show?

There is a trend towards increasing solar irradiance over the period of record.

Now look at the “Total Solar Irradiance Reconstruction (W/m2) & Temperature” graph which overlays global mean land-ocean temperatures (°C) in yellow onto the “Total Solar Irradiance Reconstruction (W/m2)” graph. The black line superimposed on the yellow represents an 11-year moving average of the temperature data.

1. Describe the relationship exhibited between the Total Solar Irradiance data and the temperature data.

Both sets of data have a positive slope with both increasing solar irradiance and global temperatures over the period of record.

2. Compare the solar radiance increase with the observed global temperature increase. Based on this data, what is your hypothesis about the role solar variability plays in the increase in Earth’s temperature that we are experiencing?

These data indicate that a portion of the observed temperature increase may result from increased solar irradiance, but other climate forcing factors are needed to explain the data that we see.

Now look at a close-up of “Total Solar Irradiance Reconstruction (W/m2) & Temperature” that focuses on the time frame 1880—2010. The blue line indicates Total Solar Irradiance, the yellow line indicates Global Mean Land-Ocean Temperatures (°C), the black line indicates the 11-year moving average of the temperature data, the red line is the linear trend line for the TSI, and the orange line is the linear trend line for the temperature data.

1. Do these data suggest a one to one correspondence between solar irradiance and earth temperatures?

There does seem to be some correspondence between solar irradiance and global temperatures, but the 1990’s and first decade of the 2000’s show increasing temperatures while solar irradiance is at a minimum, indicating that other factors besides solar irradiance contribute to the observed increase in temperature.

2. What does the data tell you about the overall trends?

The overall trends for both sets of data are positive, but the temperature increase has a steeper rate of increase than that for solar irradiance.

3. Within the context of a discussion of global warming, could an increase in solar activity, as measured by sunspot activity, be playing a role in recent warming?

There is a general correspondence between overall gradual increased sunspot activity and increased warming. However, no correspondence is exhibited between the highs and lows in sunspot activity within the 11-year cycle and global temperatures. For example, we are currently experiencing a deep minimum in sunspot activity, while the year 2010 is shaping up to be one of the warmest years on record.

Complete your data investigation of solar forcing by reflecting on what you have learned.

What questions do you still have about the relative importance of solar forcing vs. anthropogenic climate change in explaining the warming of the Earth?

Think about the barriers you currently experience with global climate change instruction and how you might use data such as the graphs you have explored in this data activity to overcome those barriers.

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