I understand how sumif work veritically, but what if I want to sumif by rows
instead of columns?

Row A1 - AB1 are different years, 2001, 2002, etc. In row B1 - BB1, there
are numbers. I would like a formula that sums up the different years.

I would transpose the data, but it needs to be in this format to work with
another software.

Thanks in advance for your help!