ORDER BY - problem with NULL values

if I order a given year in DESCending ORDER, so that the highest values (of a given variable) for the countries are displayed at the top of the list, then actually the NULL values appear as first. Only below, I find the values ordered correctly.

Is there any way to 

   a) make the countries with NULL values appear at the bottom of the list

   b) neglect the NULL values by still allowing the countries to be displayed

?

My SQL looks like this:

SELECT 

SUM(CASE WHEN year=2002 THEN value ELSE NULL END) AS "y_2002", 

SUM(CASE WHEN year=2001 THEN value ELSE NULL END) AS "y_2001", 

SUM(CASE WHEN year=2000 THEN value ELSE NULL END) AS "y_2000", 

c.name AS name 

FROM 

aquacult_prod_marine AS d 

LEFT JOIN 

countries AS c ON c.id = id_country 

GROUP BY 

name 

ORDER BY 

y_2000 DESC

Thanks for any info...

Stef