Temperature Trend in the Wadden Sea 1
- Annual and Monthly Mean Values -
Scenario
Like every year towards the end of winter, my family makes plans on where and how to spend the coming summer holidays. Mom wants to go hiking in the mountains, my sister wants to spend the holidays by the sea, and my dad will probably feel happiest in his garten at home. Last year, we spent some rainy holidays at the Baltic Sea and that spoilt dad's mood for summer holidays altogether. 
As for me, I'd really love to go swimming in the sea. Lately, I've heard that the North Sea is becoming warmer and warmer, apparently due to the global warming. Could this be true?
While surfing in the internet, I stumbled upon temperature data about the North Sea measured at Spiekeroog by the University of Oldenburg. The measurements are mean temperatures for each month. But knowing that it's hot in summer and cold in winter, the data show very strong fluctuations. So you can't really tell from these data, whether it's really getting warmer or not. 
Time Series Station and a research vessel near Spiekeroog island.
Well, I guess I'd better take a closer look at it! I downloaded the data and attached it to this worksheet. They begin in October 2002 and last until March 2009. The columns indicate the year, the month of the year, air and water temperature and the salinity of the water.
Questions
We want to find out, whether the air temperatures and - more importantly for swimming - the water temperatures have increased since the end of 2002 to early 2009. 
1.	Contemplate on how you can compute the yearly mean temperatures out of the monthly mean temperatures. Draw a diagram showing the computed mean temperatures as points over the time axis indicating the years from 2003 to 2008. Draw a straight line somewhere between the points, keeping the distance between the points and the straight line as small as possible. Can you see a trend to higher temperatures?
2.	Suppose the yearly mean temperatures are probably not very interesting for us because we go to the North Sea every year and so we only want to know the temperatures in July. Draw a new diagram showing the temperatures in July for each year and then proceed drawing a straight line between the points. Can you see now whether it has become warmer in July over the years? 
3.	How about the temperatures in January, in April and in October? Draw a diagram for these time series, too. How do you assess the results?
4.	When it rains, rainwater dilutes the sea water thus reducing its salinity. Compute the yearly mean salinity levels and translate the results into a diagram like in Question 1. Indicate the values for January, April, July and October, as well. For the vertical axis, choose a value margin for salinity levels that will use up as much space on the paper as possible and should be able to show a trend, if there is any.

University of Oldenburg, Institute of Physics, Marine Physics, D-26111 Oldenburg, Germany
Monthly averaged Spiekeroog Wadden Sea time series data, 10/2002 - 03/2009
created 26-06-2009 rr/tb

NaN: missing data (not a number)
 
yyyy	mm	T-air	T-water	salinity	
					
2002	10	8.42	9.11	NaN	
2002	11	5.83	6.68	30.6	
2002	12	0.96	2.55	31.7	
2003	01	1.51	1.71	30.7	
2003	02	0.96	1.49	NaN	
2003	03	4.67	4.84	31.2	
2003	04	7.93	8.09	32.3	
2003	05	11.32	12.40	29.9	
2003	06	18.72	18.45	NaN	
2003	07	18.67	19.69	31.6	
2003	08	16.81	18.25	32.1	
2003	09	15.87	16.73	31.8	
2003	10	8.20	9.80	32.0	
2003	11	6.87	7.04	31.7	
2003	12	4.54	4.70	31.6	
2004	01	2.64	2.79	30.5	
2004	02	4.57	4.43	29.4	
2004	03	5.48	4.92	30.7	
2004	04	9.41	9.44	31.9	
2004	05	11.55	13.05	32.4	
2004	06	14.56	16.32	31.9	
2004	07	16.11	17.38	31.2	
2004	08	20.80	21.23	33.2	
2004	09	15.18	15.87	31.0	
2004	10	12.28	11.35	31.1	
2004	11	6.45	7.87	31.0	
2004	12	3.43	4.50	30.8	
2005	01	2.24	4.51	30.6	
2005	02	2.13	2.42	30.6	
2005	03	4.38	3.41	29.12	
2005	04	9.20	8.65	30.1	
2005	05	11.89	12.60	30.3	
2005	06	16.81	17.61	31.5	
2005	07	17.66	18.98	31.9	
2005	08	16.50	17.21	30.1	
2005	09	16.43	17.21	30.5	
2005	10	13.47	13.23	30.9	
2005	11	8.56	9.44	30.4	
2005	12	4.98	4.74	29.4	
2006	01	1.02	1.66	31.4	
2006	02	2.45	2.31	32.2	
2006	03	2.45	2.43	32.3	
2006	04	7.62	7.91	30.3	
2006	05	12.84	12.68	31.2	
2006	06	15.44	16.12	30.4	
2006	07	20.68	21.12	32.0	
2006	08	17.65	19.29	30.5	
2006	09	18.27	17.91	29.8	
2006	10	13.99	14.52	30.8	
2006	11	9.66	9.56	31.1	
2006	12	7.85	7.46	29.5	
2007	01	7.12	6.62	29.1	
2007	02	5.14	4.90	30.6	
2007	03	8.32	7.31	29.6	
2007	04	11.39	11.18	31.6	
2007	05	13.05	14.45	31.5	
2007	06	17.30	18.19	31.8	
2007	07	17.10	18.23	31.3	
2007	08	17.73	18.55	31.1	
2007	09	14.75	15.27	30.8	
2007	10	11.28	12.07	30.5	
2007	11	7.64	7.75	30.5	
2007	12	4.89	5.23	29.8	
2008	01	5.66	4.49	29.2	
2008	02	5.49	4.89	29.0	
2008	03	5.75	5.77	30.2	
2008	04	8.40	8.67	30.7	
2008	05	14.04	14.27	32.9	
2008	06	16.24	17.55	32.6	
2008	07	18.15	18.67	31.3	
2008	08	17.73	18.61	31.8	
2008	09	15.04	15.86	31.5	
2008	10	10.95	11.76	30.4	
2008	11	7.80	7.98	30.9	
2008	12	3.44	4.02	30.9	
2009	01	1.56	1.70	31.3	
2009	02	2.69	1.94	31.5	
2009	03	5.52	5.13	30.1